Footage appeared to show the 22-year-old touching something in his bag during a changeover at the O2 Arena, but when asked at a post-match news conference whether he had been using his phone, Zverev said he did not even have the device with him.
"I don't know what they saw, but it was definitely not a phone," the German said after his 6-3, 6-2 defeat in the afternoon session.
"My phone was in the locker room. I always leave it there."
Was Zverev breaking rules by checking his phone? He says no. Judge for yourselves pic.twitter.com/e5NFazgOkM— Chris Latchem (@chrisdotlatchem) November 14, 2019
When asked what he might have been touching, Zverev said: "I mean, a water bottle? Empty water bottle, maybe?"
Rules dictate that players cannot use any electronic devices during matches, but the ATP issued a statement on Thursday, confirming there had been "no breach of ATP rules concerning Alexander Zverev".
The defending champion opened his account at this year's ATP Finals with a straight-sets win over world No. 1 Rafael Nadal, and next faces US Open finalist Daniil Medvedev -- who has lost both of his contests at the O2 in what is his debut at the tournament -- in his final group match.