Stan Wawrinka says Nick Kyrgios was "disrespectful in a way I could never even imagine" after the young Australian was caught on camera sledging his Rogers Cup opponent.
As the pair changed ends during their second-round tie in Montreal, Kyrgios made a remark linking his fellow Australian and good friend Thanasi Kokkinakis to Wawrinka's girlfriend - believed to be Croatian tennis player Donna Vekic - then added, "Sorry to tell you that, mate."
In his on-court interview following the match, Kyrgios was told the microphones had picked up his slur.
"He was getting a bit lippy with me," Kyrgios replied. "Kind of in the heat of the moment. I don't know. I just said it."
Wawrinka took to Twitter to express his annoyance.
"So disappointing to see a fellow athlete and colleague be so disrespectful in a way I could never even imagine," he wrote.
"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."
Wawrinka was trailing 4-0 in the deciding third set when he pulled out with a back complaint.