World number four Nikolay Davydenko has been fined $2,000 (£976) for not trying hard enough during his defeat by Marin Cilic at the St Petersburg Open.
Davydenko had been the top seed in St Petersburg
The ATP said he had been guilty of "lack of best effort".
The umpire warned Davydenko about his conduct in the final set of his 1-6 7-5 6-1 defeat. The Russian later claimed his "legs were dead".
Davydenko is being investigated by the ATP over a match in August that featured irregular betting patterns.
Online betting exchange Betfair voided bets on that match, in Poland, between Davydenko and the 87th-ranked Argentine Martin Vassallo Arguello.
It finished when the Russian withdrew with a foot injury having easily won the first set.
But on Thursday night, Davydenko protested his innocence over the match in St Petersburg, which Cilic won 1-6 7-5 6-1.
The Russian was warned by Belgian umpire Jean-Philippe Dercq in the final set for not trying hard enough.
Maybe my problems are psychological, maybe it's in my head
"I double-faulted to lose a game in the third set and he gave me a warning saying I was trying to lose on purpose," said Davydenko.
"I was simply shocked to hear him say that. This is just outrageous.
"How does he know what I was trying to do? I was so upset with the whole thing I started crying."
Davydenko spoke to the tournament supervisor after the match.
"The reality is that I started feeling tired. My legs were just dead by the third set. Maybe my problems are psychological, maybe it's in my head."
The Russian admitted that during his on-court exchange with Dercq he did not mention his physical problems.
"He could not solve my problem anyway, that why I first told him I was OK, but I didn't play the way I did in the first set, that's why he gave me a notice," he said.
"Later I told him that my legs had collapsed. I could not move."
BBC Radio 5 Live correspondent Jonathan Overend said it was difficult to prove if a player had deliberately lost a match.
"Davydenko says his legs felt dead in the third set hence the turnaround and if that's the case then it's not surprising he lost the next two sets," he said.
"How do you prove otherwise? That's the issue."
Davydenko's opponent on Thursday does not believe the Russian stopped trying.
Cilic, 19, said: "It was very tough for me to compete with him in the first set. He was hitting winners from almost anywhere, making no mistakes, but then he lost his concentration a little bit.
"I don't think that he was not trying. Maybe he just lost his game plan and I took advantage of that."
Concerns over Davydenko's match against Vassallo Arguello prompted the ATP to launch a wider investigation into the matter.