Tennis star Alexander Zverev has insisted he was not using a phone, despite appearing to be caught red-handed by TV cameras during his ATP Tour Finals loss to Stefanos Tsitsipas this morning.

Zverev was filmed appearing to scroll through a phone as he looked for a way to get himself back into the contest against Tsitsipas.

The video appears to show a device lighting up — and it led to a social media storm where the German 22-year-old was accused of breaking the ATP Tour's no-phone rules in plain sight.

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However, after the match Zverev denied he had used his phone during the match. Players are banned from bringing their phones and other technological items onto the court with them.

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Despite the damning video, Zverev said he didn't know what reporters were referring to when asked about what he was doing as he snooped through his bag during a change of ends.

He said he may have simply been fidgeting with a water bottle.

"It wasn't a phone," he said.

"My phone was in the locker room. I don't know what it was. Maybe a water bottle."

Further claims on social media suggested Zverev could have been looking at a Dexcom monitor, used to check his insulin level. Zverev is rumoured to have diabetes, though has never confirmed it.

Tsitsipas easily beat Zverev 6-3 6-2 to qualify for the semifinals.

Zverev, Daniil Medvedev and Rafael Nadal can still all qualify for the final four as they prepare for a Friday showdown.

The Greek tyro, 21, broke his big-serving German opponent three times.

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Tsitsipas grabbed the contest by the scruff of the neck at end of the first set, breaking Zverev in the eighth game and served out for the set.

The Greek then broke in the opening game and the fifth game of the second set as Zverev struggled to produce the same level of tennis that propelled him to victory against Nadal on Monday.

"I was really surprised by my performance today," said the sixth seed, who beat Medvedev earlier in the week and who is yet to drop a set in London on his tournament debut.

"I did everything right and once again it was not just me out there but a whole bunch of people, excited people that came to support me. It was a crew situation."

The straight forward victory, which took Tsitsipas's head-to-head record against Zverev to 4-1, was in sharp contrast to the earlier match, in which Nadal produced an astonishing comeback to beat Daniil Medvedev.