Nikolay Davydenko is at the centre of fresh controversy after losing to Marcos Baghdatis at the Paris Masters.
Davydenko was bemused by the disintegration of his serve
The world number four was told by umpire Cedric Mourier to "try your best" after the official questioned why he was serving so many double faults.
At the St Petersburg Open, Davydenko was fined $2,000 (£976) for not trying hard enough against Marin Cilic.
He is being investigated by governing body the ATP over an August match 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.
In St Petersburg last month the Russian protested his innocence and, after his 6-2 6-2 defeat on Thursday, when Baghdatis asked him "What's wrong?" he replied "I don't know".
I need to find what's the reason I cannot really serve
He served 10 double faults and was broken five times by the Cypriot in a match that lasted one hour and 13 minutes.
BBC Radio 5 Live tennis correspondent Jonathan Overend said: "Davydenko was jeered at times during this pathetic effort in defence of his title.
"He hit three double-faults in his opening service game of the second set and amazingly another three in his subsequent service game.
"At one change of ends the umpire, Cedric Mourier, asked the Russian why he was serving so badly. Davydenko seemed to shrug his shoulder as if to say, 'what can I do?'
"'Serve like me,' the umpire was heard to answer back.
"It's not unusual for players to banter with umpires at the change of ends but in the current climate the Russian should expect some serious questions."
Davydenko admitted afterward he feared getting an official warning from the umpire.
"He just asked me what was happening. I told him I couldn't explain," he said.
"I cannot serve. That was happening in St Petersburg. I don't have pain really. I have no pain in my elbow. I need to find what's the reason I cannot really serve."
Baghdatis said: "He didn't serve well but he played well, but I was not thinking about the stories and rumours about him.
"I don't know if they are true or not. I needed to be focused and play well. That's what I did."
An ATP spokesman told BBC Sport: "What was said between Cedric Mourier and Nikolay Davydenko was a normal exchange between an umpire and player and the ATP will not be taking the matter any further."