Top seed Zverev was convinced a shot from Alejandro Davidovich Fokina was out in their tense third round in the Spanish capital and he implored the chair umpire Mohamed Lahyani to leave his chair to inspect the mark.

Electronic line calling has become commonplace in recent years on the other surfaces, but clay events had largely persisted with line judges and the umpire coming down from his or her chair to examine the mark left by the ball when calls were contested.

The move to the electronic system was always likely to be a big test of the technology and Zverev is not along in suggesting the calls are not being made correctly.

World No 1 female player Aryna Sabalenka took a photo with her phone during a match in Stuttgart last week to highlight a poor call and Zverev followed her lead when he did the same in Madrid.

That earned him an 'unsportsmanlike conduct' warning from umpire, with the discontent among the game's top players over the electronic line calls on clay courts becoming a big concern for the sport as he moves towards the second Grand Slam of 2025 at the French Open, where line judges will not be replaced by technology for now.

"Honestly, I believe there was a failure in the system, I think there was an error in the system at that moment," said Zverev in his post-match press conference.

"I am a fan of the electronic system, but the ball wasÃ¢¦ it's not just a little, a millimetre in or out, it was like four, five centimetres. Honestly, I believe there was a failure in the system at that moment. That's why I addressed the chair umpire and said, 'Please come down to see this, I'm not crazy (laughing), I can see what was happening'."

"It will be interesting to see what happens now, what kind of fine they will give me, even if I'm right.

"I hope they don't fine me because obviously, in my opinion, I am completely right and should not be fined for this. This was not normal. Usually, the system is very reliable. Usually, the system has been correct so far in my experience, but what happened today, I don't know."

Zverev was also asked about the role of umpire Lahyani as he stated: "It's not the chair umpire's fault because if, as a rule he can't come down, he can't come down.

"So it's not Mohamed's fault, but I will talk to the supervisors, I will talk to the ATP, because as I said, this is not normal. For an error like this to happen, yes, one or two millimetres I understand, but four, five centimetres is not normal.

"I believe there was a failure in the system itself, and when something like this happens, perhaps the chair umpire should be able to come down, but it's not the chair umpire's fault. The referee can't do anything. What can he do? If the is rule he can't come down, he can't come down."

The marks a ball leaves on a clay court may exposing flaws in the technology, with Zverev eager to post the image he took on his phone on his Instagram page after the match.

Zverev's nervous performance against Davidovich Fokina may be a bigger concern to him to the line calling issues and the world No 2 was well below his best in a 2-6 7-6 (3) 7-6 (0) victory over his Spanish rival.

He served for the match at 5-4 in the decider, but threw in a dreadful service game and was relieved to get through to the last-16.

"The first set wasn't my best tennis but it is sport and it can change quickly," added Zverev. "I was down a set and a break and I had to fight. I am very happy with the win. Alejandro is playing unbelievable tennis, the best of his life, so I am happy to be through."

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Finished ICC Mens T20 World Cup