Nick Kyrgios was branded "unacceptable" after being caught on camera sledging Stanislas Wawrinka during a match at the Montreal Masters.
Kyrgios is one of the most colourful and controversial characters on the circuit, with his antics ranging from on-court outbursts at match officials to being accused of giving up half-way through a match at Wimbledon earlier this year.
But his exploits on Wednesday night have caused arguably the greatest controversy yet. The incident came when the Australian and Wawrinka, the world No5, were swapping ends. Kyrgios can clearly be heard saying: "Kokkinakis banged your girlfriend. Sorry to tell you that, mate.”
That is a reference to Thanasi Kokkinakis, Kyrgios’ Australian Davis Cup teammate and a close friend.
Wawrinka eventually retired through injury in the third set of the match and, while he did not comment on the incident in the immediate aftermath, he did not hold back on Twitter.
So disappointing to see a fellow athlete and colleague be so disrespectful in a way I could never even imagine.— Stanislas Wawrinka (@stanwawrinka) August 13, 2015
What was said I wouldn't say to my worst enemy. To stop so low is not only unacceptable but also beyond belief.— Stanislas Wawrinka (@stanwawrinka) August 13, 2015
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...— Stanislas Wawrinka (@stanwawrinka) August 13, 2015
... for this and stands up for the integrity of this sport that we have worked so hard to build.— Stanislas Wawrinka (@stanwawrinka) August 13, 2015
Kyrgios - who has recently taken on former Wimbledon champion Lleyton Hewitt as a coach - refused to apologise for his behaviour in an interview directly after the match, saying Wawrinka had started their war of words.
He said: “He was getting a bit lippy with me Kind of in the heat of the moment. I don’t know. I just said it."