Behind the Scenes of Fairness: When Tennis Judges by Data

On the centre court, a serve lands close to the baseline. The line judge raises a hand, the call of "out" rings out, and half the stands are already rising to their feet. Then the electronic board flashes "CHALLENGE". Fourteen thousand people sit back down. On the big screen, the ball's trajectory is rebuilt frame by frame, slowed until you can see the grains of chalk fly up as it strikes the ground. At last a small bright point appears, roughly two millimetres off the line. The word "IN" appears. The crowd erupts. Nobody in the stadium could see those two millimetres with the naked eye. No official, standing at the best possible angle, could tell the difference. But a system of dozens of cameras, a rendering algorithm, and a person sitting in a control room could. In twenty-five years of watching tennis, I have learned one thing: most of the decisions that shape a match are not made where the audience is looking. They are made backstage, by people who never appear on the scoreboard. There are offside errors nobody sees, but the camera never blinks. In tennis, that "offside" goes by other names: a ball on the line, a serve that clips the net, a double hit. And as in football, the smallest moments decide the largest fates. For more than a decade, tennis has undergone a quiet but total shift: the power to judge has moved away from human eyes and toward electronic systems. Hawk-Eye emerged in the early 2000s, initially serving only television — redrawing the ball's path so viewers could understand what had just happened. Then it became a challenge tool, letting players demand a review. More recently, many major tournaments have switched entirely to Electronic Line Calling (ELC) — automated line-calling with no line judges at all. The first landmark was the 2026 US Open, which ran without line judges across all courts, partly to reduce the number of people on site during the pandemic. The 2026 Australian Open followed and made ELC the standard on its show courts. By 2026, Wimbledon — regarded as the living museum of tennis tradition — announced the end of the line-judge era after nearly a century and a half, moving to electronic line-calling. That was a symbolic decision, because Wimbledon has always been the slowest place to change. This is a revolution in human terms. At many tournaments, entire teams of line judges — people who once stood along the court, bending toward every shot, raising a decisive hand — have vanished from the sidelines. The call of "out" now comes from a speaker, decided by an algorithm. People have withdrawn from the line, but they have not disappeared entirely: they have moved into the control room, where there are screens, data, and responsibility. I was once one of those people. For years as a VAR assistant, I grew used to sitting in a dark room, watching a screen, waiting for the moment when a single frame could overturn everything. In 2026, aged thirty-two, working as a VAR assistant at an AFC Cup group-stage match, I spotted that an opposition striker had been offside by 0.3 metres before scoring an equaliser. I sent a signal to the refereeing team. The goal was disallowed. The home side won 2-1. Nobody on the coaching staff knew I had intervened. I found that offside at two in the morning, after everyone had gone home. That experience taught me that technology does not replace people; it only moves where they stand. And in tennis, that new position is far more interesting than it looks. First, the technical side. An automated line-calling system does not merely "see" the ball; it models it. Dozens of cameras record the ball's position at several hundred frames per second, then software interpolates the trajectory between frames and computes the bounce point. Here is what few people understand: the error does not vanish when the number appears on screen. It is merely hidden by the resolution of the simulated image. In tennis, the margin of this error matters more than in almost any other sport, because the court is small, the ball is fast, and a single point can be the deciding point. A serve at 200 km/h travels from the service line to the baseline in about half a second. At that speed, the ball deforms as it strikes the ground, and the actual contact point is not a point but a smear. The system must decide: which edge of the smear counts? That is a technical choice, but also a philosophical one. Tactically, this changes how players play. Knowing that a ball two millimetres from the line will be called "in", players edge closer to the lines — but also grow more cautious on the big points. On a fast surface, where the ball skids low and quick, the advantage goes to whoever dares to hit close to the line; on clay, where the ball bounces high and slow, the safety margin is larger, and the system's precision is less often pushed to extremes. Surface adaptability therefore becomes a variable that data can measure. Players who serve well on hard courts but poorly on clay tend to show large gaps in their rate of unreturned serves. When line-calling is accurate to the millimetre, those gaps stand out more clearly, because there is no longer the "luck" of a wrong call to hide them. Then there is the psychological factor on the decisive points. In tennis, "clutch" — the ability to play well in the crucial moment — is the hardest thing to measure. But data from line-calling helps measure it better. When we know exactly where every ball landed, we can calculate: on break points, do players tend to aim closer to the lines or safer? And does that tendency change by set, by score, by opponent? This is where data becomes tennis's second language. A modern line-calling system answers the question "in or out", and at the same time generates an enormous amount of data: serve speed, placement, spin, rally length, first-serve points won, second-serve points won. From these numbers, people build heat maps of serve placement, charts of shot tendencies, and even predictive models. But here is the crucial point I want to stress: data does not tell the story by itself. It only tells it when someone knows how to ask the right question. I have seen too many reports dozens of pages thick, full of beautiful charts, that cannot answer the one question a coach actually asks: "So what do I do differently tomorrow?" Based on my experience watching matches across many seasons, I have found that fitness and form data matter more than ever when players compete continuously across continents. A player can play well in the first three rounds, then collapse in the fourth through accumulated fatigue. A data system, read correctly, can warn of that before it happens: serve speed dipping, second-serve points won falling, steps per point rising. That is the true value of data: not to show off precision, but to detect early signals. In medicine, this is called an "early warning signal". In tennis, it is the difference between entering a quarter-final with fresh legs and entering it with a body that has run out of battery. Alongside that is the story of tournament systems and schedules. Not every tournament can afford an electronic line-calling system. A Grand Slam can spend millions on technology; a Challenger or ITF event cannot. The result is a two-tier system: at the top, decisions are made by machines; lower down, they still depend on human eyes — with all the error that entails. This inequality has direct consequences for ranking points and prize money. A lower-tier player can lose a crucial point to a wrong line call, while a top-tier player is protected by technology. Fairness, in the end, is also a kind of commodity — and not everyone can afford it. On scheduling, the dense calendar of the annual season raises questions of rationality. Constant surface switching — from hard to clay to grass within a few weeks — is a major challenge for the body. Line-calling does not solve this, but the data it generates can help identify who is under the greatest strain. Looking more broadly, the tennis landscape is shifting by generation. The older generation, with its experience and composure, still holds the biggest titles. The generation in its prime pushes into the middle. And the young generation, raised alongside data, treats numbers as a natural part of the game. The interesting thing is that the young generation is no longer afraid of data. They grew up with smartphones, with fitness trackers, with stroke-analysis apps. To them, a computer system calling the lines is not an intrusion but an obvious fact. Meanwhile, some veteran players still remember the feel of matches without technology, where the umpire's decision was final and unappealable. At the level of rules and governance, the biggest question is: who is responsible when the system is wrong? When a line judge errs, we can reprimand, retrain, or replace them. When an algorithm errs, what do we do? Nobody can punish a line of code. This is the accountability gap that tennis, like many other sports, has yet to fill. The rules around challenges — how many reviews are allowed, when they are permitted, when they are not — are also a sensitive area. At tournaments without ELC, the challenge remains a tactical tool: used at the right moment to break an opponent's rhythm, or to catch a breath. At tournaments with ELC, that right disappears, and the match flows faster and more continuously — but with fewer breaks. In team and player management, the role of analysts keeps growing. Every top player now travels with a team of coach, fitness specialist, doctor, and often a data analyst. That person sits in the stands, takes notes, and sends signals down via headset or hand signs. But there is a fragile boundary. When data intrudes too deep into the locker room, it can erode instinct. I have seen players lose confidence because of too many numbers in their heads: they start calculating instead of feeling, analysing instead of reacting. And in tennis, where everything happens in a split second, reaction is always faster than calculation. The risk, therefore, lies in injury, in fitness, and also in dependence. When we trust data absolutely, we forget that data is a photograph of the past, not a prophecy of the future. No matter how good a predictive model is, it cannot calculate the moment a player suddenly rediscovers inspiration, or an opponent suddenly plays as if by magic. There is another, subtler risk: the risk of "flattening" the match. When every decision is standardised, when every error is eliminated, tennis loses part of its appeal — the part of uncertainty, of argument, of human moments. That is a price not everyone wants to pay. On the media front, the story is complex too. On one hand, the public welcomes technology because it brings fairness. On the other, whenever a decision is controversial, they demand the return of the "human umpire". This is an inherent contradiction: we want the precision of machines, yet we miss the warmth of people. I have seen this in my own work. When a VAR decision is right, nobody mentions it. When a decision is wrong, the whole world mentions it. That is the nature of backstage work: fairness, when correctly enforced, becomes invisible. And the invisible earns no praise. The biggest mistake is not blowing the whistle, but failing to own your whistle. In 2026, at the World Cup round of sixteen, I failed to spot a handball in the penalty area, which led to a penalty. I blamed myself for three weeks, quietly re-watching all 64 matches of the tournament, taking notes on every incident. I did not share that feeling with colleagues. But that experience taught me this: whoever makes judgement calls must have the courage to admit error in public. At a deeper level, judging technology is creating a new value chain. Hawk-Eye is owned by Sony, and its systems serve not only tennis but football, cricket, and many other sports. The data gathered from matches becomes raw material for broadcasting, for betting, for commercial analysis, and for the teams themselves. For tournaments, investing in technology is a way to raise their brand. A Grand Slam with a modern line-calling system looks more professional, more trustworthy in the eyes of sponsors and viewers. For equipment makers, stroke data is an opportunity to develop better rackets, balls, and shoes. For media platforms, data is content to attract views. But at the very bottom — where young players and small tournaments survive — technology remains a distant dream. There, a wrong call can still destroy the career of someone who has not yet become famous. And it is precisely there that the need for fairness is most urgent. This is where I must say what many are reluctant to say. The pursuit of absolute precision can be a dangerous illusion. For even the best system has error, and hiding that error behind a crisp simulated image does not make it disappear. It only makes viewers believe that machines cannot err — and therefore leaves no room for healthy doubt. The paradox is this: the more we trust machines, the more easily machines can lead us astray. An algorithm trained on past data will reproduce the biases of the past. If line judges, over decades, tended to call "out" on balls near the line, then the training data carries that tendency too. Machine learning is not inherently fair; it is fair only if taught to be fair. This is the biggest blind spot of the technology revolution in sport. We focus on eliminating human error, but say little about system error. We praise the "camera that never blinks", yet forget that it was a programmer who decided where the camera looks and what it ignores. A single millimetre changes a player's fate; I have learned to live with that. But I have also learned that behind every millimetre there is always a person who must be accountable — whether umpire, technician, or analyst. Technology does not erase responsibility; it merely shifts responsibility elsewhere, more discreetly. Emotion and rules, in the end, are not opposites but complements. Rules give us a framework; emotion gives us a reason to respect that framework. A match without rules is chaos; a match with only rules and no emotion is a machine. Tennis, at its best, is the fragile balance between the two. Looking ahead, I believe the trend toward electronic judging is irreversible. In ten years, most major tournaments will run without line judges. But I hope that, alongside this, tennis will build a more transparent mechanism for system error — a way to disclose the confidence margin of each decision, rather than presenting it as absolute truth. I also hope that smaller tournaments will gain access to this technology at lower cost, so that fairness is no longer a privilege of the wealthy. And I hope that, however far machines advance, tennis will keep a place for people — not on the line, but elsewhere: in making the final call, in taking responsibility, and in admitting that no system is perfect. The umpire is the only person on court not allowed to take sides — and I stand behind them. Whether that person is an umpire of flesh and blood, or an algorithm running on dozens of cameras, I still believe the justice of the match deserves protection. Even when it is invisible. Even when nobody remembers the name of whoever kept it.