Tennis adopted electronic line calling on clay and created a ball-mark monster

“Can you explain this?” inquired Alex Eala following her loss to Iga Świątek at the 2025 Madrid Open.

Eala, the 20-year-old rising Filipino tennis star, wasn’t referring to her three-set defeat. Instead, her frustration stemmed from a ball mark on the red clay court. Early in the contest, Świątek served a ball that, according to the physical mark on the clay, appeared to be out. However, no ‘out’ call was made, as the electronic line calling (ELC) system deemed it to be in.

“I mean, the mark is the physical, it’s the physical proof,” Eala stated.

A year later, at the very same tournament, it was Australian Open champion Elena Rybakina who experienced a similar situation. She confronted the chair umpire, indicating a ball mark from a Zheng Qinwen serve was clearly outside the line.

“This is not a joke. The system is wrong. This is not a joke. It is not touching. It is absolutely wrong,” Rybakina asserted, prior to ultimately winning the match in three sets.

“It’s kind of a stolen point. I understand it was her serve and she was serving ⁠really ​well, but it’s really frustrating,” she later conveyed to reporters.

Both Rybakina and Eala highlighted an increasingly common predicament for tennis players on clay courts: being instructed to distrust their own visual assessment. For many decades, clay court tennis has depended on human line judges to call ball bounces and chair umpires to physically examine the marks left on the red dirt. With the introduction of ELC at some events, the system’s rulings are now frequently at odds with what players visually perceive.

Some players have even resorted to photographing ball marks they believe substantiate their arguments. In Madrid last year, top seed Alexander Zverev photographed a mark with his phone after a shot from Alejandro Davidovich Fokina, which he believed was out, was called in. A week earlier in Stuttgart, Germany, women’s world No. 1 Aryna Sabalenka did likewise, documenting a shot she felt was in but had been called out. In Sabalenka’s case, ELC was not in use, and she was disputing a human line judge’s call. Both players incurred code violations for unsportsmanlike conduct.

During his post-match press conference in Madrid, Zverev maintained that there was a “defect in the system” at that specific moment.

“I will talk to the supervisors, I will talk to the ATP because, as I said, this is not normal. For a mistake to happen like this, yes, one or two millimeters I understand, but four, five centimeters is not normal.”

With the simple bounce of a ball, tennis finds itself grappling with a fundamental dilemma that probes the very essence of human perception. When confronted with conflicting information from two distinct systems of knowledge, which one should an individual ultimately credit?

By 2026, Hawk-Eye ELC is implemented at three of the four Grand Slams and across all ATP and WTA tour events above the 250 level, including the Madrid Open. The Australian Open transitioned from line judges to this system in 2021, followed by the U.S. Open in 2022. Wimbledon adopted it last year. The French Open, however, has maintained its use of human line judges, continuing to rely on ball marks in the clay to determine whether shots are in or out.

While tennis balls leave marks on all playing surfaces, these are virtually imperceptible to the naked eye on hard and grass courts, though on grass, chalk can occasionally erupt if a ball lands directly on or very near a line. On clay, however, the visible traces left by the ball can generate discord when they contradict technological determinations.

ELC systems typically operate with a margin of error of a few millimeters, yet ball marks on clay can be misleading. Clay is a “live” surface, influenced by variations in the amount of red brick dust across the court, prevailing weather conditions, and the trajectory of the shot. Tennis balls compress upon impact, creating marks of diverse shapes and sizes that don’t always provide an accurate representation of the actual point of contact. Even systems touted as 100 percent accurate, such as Foxtenn, which employs cameras to visualize the ball’s bounce, can be affected by these complexities.

Before ELC was rolled out on clay courts, the ATP, in consultation with the WTA, funded and commissioned an educational video specifically designed to clarify these optical illusions for both players and spectators.

ATP players received this video, and the WTA disseminated it through its channels, briefing its athletes on some of the peculiarities of ELC on clay. Nevertheless, this hasn’t prevented some players from expressing intense frustration over perceived injustices, even a year later. Furthermore, instead of showing the ball’s bounce at court level, television broadcasts continue to use the top-down Hawk-Eye replay—identical to those used on grass and hard courts—which offers little reassurance to players who clearly see a mark that doesn’t align with the electronic ruling.

Ball marks on clay can appear differently depending on surface conditions. This is one of the key reasons ELC Live was introduced across the Tour.

Learn more 👇 https://t.co/IeVUC9vavY

— ATP Tour (@atptour) April 28, 2025

Last year in Madrid, the ATP Tour’s X (formerly Twitter) account even replied to a post by Tennis TV, an ATP-owned media entity, regarding the Zverev incident. The ATP stated it was unaware of any defect, while Hawk-Eye offered no comment when requested.

In other match situations, players have sometimes chosen to let the physical ball marks dictate the outcome. Last year, two-time Grand Slam singles and mixed doubles champion Victoria Azarenka had a serve called long during a doubles match against Demi Schuurs and Asia Muhammad. However, Schuurs and Muhammad consulted the ball mark and, exhibiting sportsmanship, awarded Azarenka a first serve.

While this type of gracious conduct is widely lauded in tennis, in this context it carries broader philosophical implications for officiating. If ELC is the definitive determinant of whether a ball is in or out, then physical ball marks should logically play no role in any decision-making process. For players, this necessitates unlearning years of ingrained mental habits—not just physical strokes—by consciously disregarding not only the tangible evidence before their eyes but also the traditional system of knowledge they’ve long relied upon to comprehend their sport.

This disparity has been evident to spectators for a considerably longer period. A notable early instance for the introduction of ELC was the 2004 U.S. Open quarterfinal between Serena Williams and Jennifer Capriati. Numerous shots by Williams were called out, despite Hawk-Eye replays clearly showing them as in—however, this technology was only available to television viewers and not utilized for live match officiating.

From 2007 to 2024, Wimbledon, alongside the U.S. Open and Australian Open before line judges were removed, implemented a Hawk-Eye challenge system. Players could dispute calls they disagreed with, but they were limited to three incorrect challenges, preventing them from scrutinizing every close decision. This often led to viewers seeing replays of balls called one way, which were in fact the opposite.

In a sport already fraught with numerous mental hurdles, it was inevitable that players would occasionally become agitated when instructed to distrust their own sight—even after watching an educational video explaining the potential causes of such discrepancies.

In 2024, Zverev, in a broader outburst during a Shanghai match, attributed his two Grand Slam final defeats that year to umpires. During the pivotal fifth set of that year’s French Open final, a second serve from Carlos Alcaraz was called out by a line judge, but the chair umpire intervened and overruled the call. The Hawk-Eye television replay subsequently showed the ball to be out—though it fell within the technology’s approximate 3mm margin of error.

Absolute certainty can prove to be an illusion, particularly when the underlying premise of players’ arguments against ELC is that their own eyes can more accurately track a small, yellow object traveling at speeds exceeding 100 mph than a sophisticated system specifically engineered for that sole purpose.

Taylor Fritz, the American world No. 4, remarked that players’ ability to adapt can vary based on their tennis background: “For someone like me, who’s not a clay-court player, stopping points and looking at marks mid-point, trying to see if it’s in or out, is so much tougher to do than someone who’s played on clay forever,” he informed reporters in Madrid last year.

“So I really appreciate the ability to just play until we hear a call. If no call comes, we understand the point is still live. There’s no ambiguity, and regardless of whether a call might be missed, I don’t mind, at least there’s consistency.”

Sabalenka, who questioned a ball mark in Stuttgart, supports ELC because it removes the subjective, emotional element of an umpire potentially reluctant to acknowledge an error. “When it’s a minor mistake or perhaps not even a mistake, I prefer the Hawk-Eye system over a human referee because sometimes referees can be quite hesitant (and prefer not to) approach and confirm their own error,” she explained to reporters.

Prominent players Madison Keys, Alex de Minaur, and Elina Svitolina conceded that while frustrations with ELC are inevitable, they are preferable to the previous dynamic of disputing an umpire’s inspection of a ball mark.

“I always detested it when they would come down and gesture with their fingers,” Keys mentioned in an interview in Madrid last year.

“This isn’t going to turn out the way I want it to.”

Fils holds a contrary perspective.

“We need to revert to conventional line judges and trust the umpire, because currently the umpire’s role is diminished,” he told reporters in Madrid that same year. Historically, players have also vehemently protested umpires making decisions based on what they believed to be the incorrect ball mark.

Given that the disparity between ball marks and ELC only manifests on individual shots—a tiny fraction of the total points played throughout an event—it’s easy to overlook that players are being asked to fundamentally redefine their understanding of the sport. It may prove challenging for them to disregard ball marks from their perceptions, even when they are aware of their potential for illusion.

Świątek, who commented after her victory over Eala that “there’s no point” in players debating their preference for ELC versus umpires inspecting marks, appears to have reached her own form of acceptance.

“You might prefer playing with umpires, but when you step onto the court and electronic line calling is in use?” she questioned during a news conference. “There’s no time to adopt that kind of mindset, so I will simply accept whatever system is in place.”