Jannik Sinner is cruising through yet another Australian Open. The Italian waltzed into the quarterfinals, taking down compatriot Luciano Darderi in the fourth round with a stellar straight-set win, 6-1, 6-3, 7-6 (7-2). But early in the match, Sinner ran into a minor issue with the umpire, one that reminded fans of the same problem Carlos Alcaraz faced in his R4 clash.

During the match, the No. 2 seed was among several players spotted wearing a screenless fitness tracker on court. Moments before stepping in to face Luciano, he was asked to take it off. Later, Sinner explained the situation openly.

“Yeah, there is a certain data we would like to track a little bit on court,” Sinner said in his press conference, “It’s not for the live thing, but it’s more about what you can see after the match, and these are the data we would also like to use in practice sessions because from that you can practice with the heart rate and how many calories you burn and all these kinds of things.”

However, Sinner didn’t make a big deal of the situation and said he was already looking at some alternatives to track data.

“It’s the vest, but for me it’s a bit uncomfortable because you feel like you have something on the shoulders, and it’s a bit different,” Sinner continued. “But rules are rules, and I understand and won’t use it again.”

The device in question was a WHOOP wristband. It’s a sleek, screenless fitness tracker that monitors biometric data like heart rate, recovery, and physical strain. It also tracks sleep, stress, and overall fitness, giving players deep insights into their conditioning beyond match play.

Earlier in the tournament, Carlos Alcaraz faced the same issue. He, too, was asked to remove his WHOOP band during his match against Tommy Paul.

According to International Tennis Federation guidelines, WHOOP 3.0, 4.0, 5.0, and MG models are classified as approved player analysis technology, but not for Grand Slam use.

The Spaniard later addressed the situation, taking responsibility for the slip.

“These are the tournament’s rules, the ATP’s, the ITF’s,” Alcaraz told the press after defeating Paul. “You can’t play with it. It’s something that helps you take better care of yourself, to monitor your recovery, your training load… but, well, I haven’t been able to use it in matches. It’s fine. You take it off and keep going.”

The ATP, which runs the men’s tour but not the Grand Slams, gave players the green light to use the technology during matches in 2024. On the women’s side, the WTA has already embraced it, encouraging its pros to bring a data-driven edge to competition.

However, even Aryna Sabalenka was asked to remove her fitness tracker before her AO first-round match.

WHOOP founder Will Ahmed called the situation “ridiculous” on social media.

“Let the athletes measure their bodies,” Ahmed urged. “Data is not steroids.”

For Sinner, that data may have been especially useful under the fierce Australian sun. The Italian had cramped up in his previous match on Saturday against Eliot Spizzirri. To clear the air, his coach later spoke up about what went wrong.

Jannik Sinner’s coach explains what happened during his R3 match

The Melbourne sun showed no mercy, with temperatures soaring to 40°C. The World No. 2 felt the full force of it in his third-round clash, pushing through cramps and fatigue as the heat sapped his energy. But he refused to give in, cutting rallies short and conserving just enough fuel to keep fighting.

The brutal heat tested players to their limits, but for Jannik Sinner, it became a survival mission, as he looked close to breaking at times. Yet, as his coach Darren Cahill later explained, there was method behind the struggle.

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“It’s clear that Jannik was very lucky with the timing of the roof closure,” Cahill said on Monday. “Everyone on the team knew that, at some point, the match would be paused to deploy the retractable roof. Our goal was for him to hold on as best as he could until the end of the third set.”

Sinner admitted he caught a lucky break when the roof finally closed, giving him the breather he desperately needed, even revealing his thoughts later.

“I believe that I’m someone who tries to put tennis in the highest priority, and I know that I’m doing that for years now,” Sinner said. “In the back of my mind, I know how much I work. I feel well prepared, even if some problems could happen potentially on the court.”

Now, the 22-year-old is charging into the quarterfinals, where he’ll face Ben Shelton once again. Last year, he took down the American in straight sets in the semifinals before lifting the trophy for the second time.

The timing for the quarterfinal is yet to be confirmed. Indoor conditions will surely help Sinner, but Shelton has said he is “okay with either one.”