Tennis star Alexander Zverev has become the latest player to question the accuracy of electronic line-calling (ELS) systems, specifically on clay courts. During a Madrid Open match against Alejandro Davidovich Fokina, Zverev disputed a call made by the Hawk-Eye system, which deemed a ball "in" despite the visible mark on the court suggesting it was out.

The incident highlights the challenges of implementing ELS on clay courts, which are unique in that they leave a mark when a ball bounces. This mark has traditionally been used as clear evidence of whether a ball was in or out. However, with the increasing use of ELS, players are now relying on complex software to make these calls.

Zverev's frustration was evident as he pointed out the discrepancy between the ELS call and the ball's mark, saying "The machine is not working... Look at this mark... please come down." He even took a photo of the mark on his phone, which resulted in a warning for unsportsmanlike conduct. Other players, including Victoria Azarenka and Eva Lys, have also taken photos of disputed shots during the tournament.

The Association of Tennis Professionals (ATP) has been replacing human line judges with ELS across major tournaments, with the French Open being the only exception. Hawk-Eye, one of the companies behind this technology, uses a set of cameras to predict where a ball is headed by analyzing its speed, spin, and skid with a three-millimeter accuracy.

However, the company's system assumes where a ball will bounce before it arrives, which can be affected by various factors on clay courts, such as the wetness or dryness of the surface, as well as the amount of clay on the court. This can make some marks deceiving to the human eye, and potentially lead to incorrect calls.

ATP supervisor Cedric Mourier acknowledged the challenges of working on clay courts, saying "Clay is a very different surface — probably the most difficult surface to work on... Interpreting a mark is subjective, the reading of the mark very complicated. And this will be obviously, taken away by the ELS live."

The incident raises questions about the reliability of ELS on clay courts and whether it can accurately replace human line judges. As the technology continues to be implemented across major tournaments, it's likely that we'll see more disputes between players and the system. The ATP will need to address these concerns and ensure that the technology is accurate and reliable, especially on clay courts.

The use of ELS is not limited to tennis, with other sports such as cricket and football also adopting the technology. As the technology continues to evolve, it's essential to ensure that it is accurate and reliable, and that it doesn't compromise the integrity of the game.

In the meantime, Zverev's incident serves as a reminder that even with advanced technology, there is still room for human error and interpretation. As the tennis world continues to adapt to ELS, it's crucial to strike a balance between technology and human judgment to ensure fair play.