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.

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Wawrinka had withdrawn from Wednesday's Rogers Cup match against Kyrgios while down 0-4 in the third set, but not before Kyrgios' on-court claim that Thanasi Kokkinakis slept with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

Further sanctions a possibility: ATP

Kyrgios was hit with an additional $US2,500 ($3,390) fine on Friday for unsportsmanlike conduct relating to a comment made to a ball person during the match, the ATP said in a statement.

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Tennis's governing body also served him with a 'Notice of Investigation' to determine whether Kyrgios's actions also constitute a violation for the Player Major Offence provisions under 'Aggravated Behaviour or Conduct Contrary to the Integrity of the Game'.

Such an investigations opens up the possibility of more sanctions for the player, such as additional monetary fines and/or suspensions from ATP events.

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.

"If you're going to come and openly, it's not even sledging, that's openly slandering someone after a match, then you can expect a bit of sledging back."

Christos was suddenly cut off from the show with the radio station saying an inappropriate comment had been made.

Tennis Australia said in a statement Friday that the ATP's actions were appropriate, while promising to support Kyrgios.

"Nick Kyrgios has apologised to all involved for his comments on court in Montreal and deeply regrets his actions," the statement said.

"Tennis Australia is working closely with Nick and his team to provide the best possible support; we understand how important it is to help educate and influence him both on and off the court.

"Appropriately the ATP, as the governing body in this case, has issued the maximum fine and continues to review and work through this situation with Nick and his management."

Kyrgios has 'no excuse': Djokovic

Sorry, this video has expired Djokovic says Kyrgios deserved to be fined

World number one Novak Djokovic said that Kyrgios deserved to be punished.

"I'm not the one who should judge the reaction but in my opinion it was completely unnecessary to comment something like that," said Djokovic.

"There is no excuse for what he has said. He was fined, he deserved it.

"I think he's going to learn a lesson in a hard way. Hopefully this won't happen to him anymore."

Djokovic said that Kyrgios, fast earning a reputation as the sport's bad boy, remains one of the potential future stars of the sport.

"He's young, he's definitely one of the players that everybody's looking to.

"His quality of tennis is really remarkable for his age. He's doing very well.

"I've never had any problems with him personally. Off the court we had nice chats, practised with each other."

Former women's champion and legend of the game Chris Evert echoed the Serbian's sentiments.

"A hurtful, offensive comment like that is not a good reflection on Kyrgios or the sport," she wrote on Twitter.

Two-time Australian Open champion and former world number one Victoria Azarenka also rued Kyrgios' choice of words, saying "you have to respect your opponent. Everyone has to ... always."

But Australia's Davis Cup captain Wally Masur believes the controversy has been blown out of proportion.

"When I actually saw the incident, what was fairly obvious is that Nick had his back turned to Stan, he was 90 feet away, and he mumbled it under his breath, I mean no way was he actually being confrontational," he said to Fox Sports News.

"It's just a case of the court-side microphone, which is pretty amazing what it picked up, picked up the audio.

"It wasn't the scenario I had in my mind when I read the headlines."

French Open champion Wawrinka had after the match demanded action by tennis authorities over the "beyond belief" comments.

"What was said I wouldn't say to my worst enemy. To stop (sic) so low is not only unacceptable but also beyond belief," he tweeted.

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.

In his post-match press conference, Wawrinka told reporters he had confronted Kyrgios, who has "big problems", in the locker room after the match.

The ATP released a statement which said, in part: "Further details of the fine will be made after Kyrgios has been notified."

ABC/Wires