Nick Kyrgios could face action after telling Stan Wawrinka that Thanasi Kokkinakis "banged your girlfriend" midway through their match at the Montreal Masters.
The Australian tennis player's trash talking was picked up by a court-side microphone at the hard court event, which is one of the major tournaments in the build up to the US Open.
Midway through the second set of their match, Kyrgios alleged that teenage Australian player Kokkinakis had slept with the girlfriend of the reigning French Open champion.
"Kokkinakis banged your girlfriend," he said. "Sorry to tell you that, mate".
Wawrinka hit back at Kyrgios on Twitter and urged the ATP to take action, which seems likely.
"So disappointing to see a fellow athlete and colleague be so disrespectful in a way I could never even imagine.
"What was said I wouldn't say to my worst enemy."
Stan Wawrinka at the Montreal Masters
He continued: "To stop [sic] 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."
Donna Vekic, the Croatian player understood to be dating Wawrinka
Wawrinka, who is reportedly dating fellow professional Donna Vekic, progressed in the tournament after Kyrgios retired despite leading 6-7, 6-3, 4-0.
Tennis stars get glamorous at the Wimbledon Champions' Dinner 2015
Kyrgios is no stranger to controversy. At Wimbledon earlier this year he was given a code violation for swearing, hugged a ball-boy, was repeatedly heard swearing on court, bounced his racket into the crowd and had an argument with an umpire over changing his socks.
