In April 2023, the ATP mandated that Electronic Line Calling (ELC) will be used on all courts across all tournaments, beginning in 2025. Historically, line judges (the guys / gals standing at the back of the court with their hands behind their backs) have been responsible for calling close “outs,” but this has changed with new developments in AI. The Australian Open adopted ELC in 2021, while the US Open did so in 2022. As you could imagine, the two big holdouts were Roland Garros and Wimbledon, with the organizers citing decades of tradition and a desire for human control as reasons to maintain line judges. Personally, I’m going to miss moments like this.

Line Judge’s Reaction after Being Praised by Alcaraz for Close Call
The goal of ELC is to eliminate “human error,” calling lines as accurately and quickly as possible to ensure the fairest and smoothest game. Generally, about 3% of points were challenged prior to ELC. In other words, for every 33 points played (roughly the equivalent of up to 8 games), only one point involved a ball that was so close to the line that its position was questioned. While the outcomes of these challenges could certainly be important (especially for game momentum), we are talking about a relatively small part of the game.
Previously, players had the ability to use 3 challenges per set. Going forward, each and every point will be “challenged” automatically, with the outcomes of these challenges being final, whether or not the ball was actually in…
Right now, you might be thinking, What? That makes no sense. How could the combination of 10-18 cameras and AI ever be wrong? I would argue that Hawk-Eye is right 99% of the time. Even so, there are still many examples of how calls made by man vs. machine can be inconsistent. Let’s consider a recent example from the ongoing 2025 Aussie Open – just take a look at this clip!
The official rule per the ITF is, “If a ball touches a line, it is regarded as touching the court bounded by that line.” Seems reasonable enough, right? However, you saw the side angle of that ball – would you, or the line judge, say that it “touched the line”? Is this even something that is fully objective, based on the pictures below?
Hawk-Eye vs. Side View – Is the Ball In or Out?
There are two sub-topics that are worth discussing:
Angle of perception
Compression of the ball
Let’s start with #1. When a line judge makes a call, they are generally viewing the ball from the back of the court from a “side angle” – an acute angle relative to their eyesight vs. the court. When the Hawk-Eye makes a call, it uses footage from many cameras set up at different angles around the court to triangulate an AI-generated, 3D depiction of the ball’s trajectory. If we assume that the Hawk-Eye is accurate (the tech claims accuracy to 3.6mm), then perhaps there are cases when human eyesight could lead to inaccuracy – after all, tennis balls are spherical objects, and the full diameter of the ball doesn’t make contact with the court.
The line judge is just fine when the ball makes contact next to the singles or doubles lines… but when the ball is in front of or behind the service line or baseline, they may not be at the optimal angle to make the right call. On the other hand, human eyes can certainly be accurate to less than 3.6 mm. Also, if the sport was originally designed for line judges to be making calls, does that suggest implementing tech changes the game? And if so, should the game change?
It’s unclear, but one great aspect of the game that no longer exists as of this month is that players used to be able to selectively use their limited number of challenges to call close balls and mess with their opponents’ heads – or even better, the opponent could make the challenge and be wrong, like in this case:
Regarding topic #2, tennis balls are hollow, as we discussed in the October 2024 blog post. Therefore, they compress when hit by the racket and when they bounce on the court. The harder the ball is hit, the more it compresses, and the greater the surface area that is in contact with the court. This compression can range from 1.4 - 1.6 inches, representing a significant, 5 mm range – notably, above the Hawk-Eye’s accuracy.
Does this mean that pros who have a harder-hitting game have a greater probability of winning challenges? And is that probability significant enough to change the outcome of a match? In a sport played on such thin margins on a professional level, I would lean towards yes. Notably, on clay courts, chair umpires check ball marks to confirm calls – the size of the mark is certainly impacted by the speed and spin of the shot. So perhaps there would be a lesser variance between how man would make calls vs. machine at tournaments such as the Monte-Carlo Masters and the Madrid Open.
Given the ELC mandate that began this year, perhaps the best way for pros to adapt is to spend a bit of time understanding how the electronic system processes data to make a call and how this may differ vs. a human. Training to hit a few centimeters closer within the bounds of the lines might just pay off one day!
What do you think – should ELC be universal, or should tournaments be able to choose for themselves?
Universal
Tournament's Decision
Works Cited
Abramitzky, R., Einav, L., Kolkowitz, S., & Mill, R. (n.d.). On the Optimality of Line CallChallenges in Professional Tennis. Stanford University. https://web.stanford.edu/~leinav/pubs/IER2012.pdf
ITF Rules of Tennis. International Tennis Federation. (n.d.). https://www.itftennis.com/media/2510/2020-rules-of-tennis-english.pdf
Tennis Ball Performance Testing. Tennis Warehouse University. (n.d.). https://twu.tennis-warehouse.com/learning_center/ballexperiment.php#:~:text=Such%20an%20impact%20compresses%20the,between%2065%20and%2071%20ms
Excellent article, Raj