Australian Nick Kyrgios has been booed off the court following his Montreal Masters loss to John Isner, after being fined for his crude on-court comment about Swiss opponent Stan Wawrinka’s girlfriend in his previous match.

The Association of Tennis Professionals (ATP) sanctioned the Australian with a $US10,000 ($13,580) fine after he apologised for his outburst on social media prior to Friday’s match, and a hostile crowd booed his entry and prompt exit from the court as he lost to Isner 7-5, 6-3.

“I would like to take this opportunity to apologise for the comments I made during the match last night (Wednesday) vs Stan,” Kyrgios tweeted before facing Isner.

He also posted on Facebook, adding: “My comments were made in the heat of the moment and were unacceptable on many levels.”

“In addition to the private apology I’ve made, I would like to make a public apology as well. I take full responsibility for my actions and regret what happened.”

Wawrinka had withdrawn from Wednesday’s Rogers Cup match against Kyrgios while down 0-4 in the third set, but not before Kyrgios’ on-court claim that Thanasi Kokkinakis slept with Wawrinka’s girlfriend.

While walking to his seat having lost the first-set tiebreak, Kyrgios was heard saying: “Kokkinakis banged your girlfriend, sorry to tell you that mate.”

The ATP said on Thursday it had taken the action against 20-year-old Kyrgios for the comment during the Wawrinka match, which was picked up by a courtside microphone.

Nick’s older brother, Christos Kyrgios, said on radio there was already bad blood between his brother and Wawrinka, dating back to the pre-Wimbledon Queen’s Club tournament.

“Stan openly said after their match at Queen’s that Nick was faking injury and that he wasn’t even sick,” he told Triple M.

The woman in question is believed to be 19-year-old Croatian player Donna Vekic, who Wawrinka was rumoured to be dating after separating from wife Ilham Vuilloud.

Kokkinakis had also been linked with Vekic after the pair played mixed doubles together at the 2014 Australian Open.