French Open champion Stan Wawrinka has branded Nick Kyrgios "unacceptable" and called for action to be taken against him after the Australian appeared to make comments about his girlfriend.
Kyrgios knocked Wawrinka out of the Rogers Cup after the Swiss retired hurt, but it was an apparent exchange at the end of the first set which has become the focal point.
On-court microphones recorded 20-year-old Kyrgios walking to the net and saying: "Kokkinakis banged your girlfriend. Sorry to tell you that mate" - an apparent reference to Australian Davis Cup team-mate Thanasi Kokkinakis.
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Speaking on court afterwards, Kyrgios claimed he had been provoked, saying: "He was getting a bit lippy with me. Kind of in the heat of the moment, I don't know. I just said it."
His reasoning has not placated Wawrinka, though, who has called for the ATP to take action.
Writing on Twitter, he said:
He continued: "What was said I wouldn't say to my worst enemy. To stoop so low is not only unacceptable but also beyond belief.
"There is no need for this kind of behaviour on or off the court and I hope the governing body of this sport does not stand for this and stands up for the integrity of this sport that we have worked so hard to build."
Controversy has started to follow Kyrgios around since his 2014 breakthrough when he knocked Rafael Nadal out of Wimbledon.
At this year's tournament he was moved to deny accusations of abusing an umpire and throwing a game, while on Davis Cup duty with his country he shouted "I don't want to be here" while on court.