MONTREAL (AFP) - Controversial Australian Nick Kyrgios apologised to Stan Wawrinka Thursday for an obscene sexual remark he made on court which sent shockwaves through tennis and cost him a US$10,000 (S$14,000) fine.

The 20-year-old made his apology just after being fined for the incident which took place during the pair's Montreal Masters second-round clash on Wednesday which Kyrgios won.

"I would like to take this opportunity to apologise for the comments I made during the match last night v Stan Wawrinka," Kyrgios wrote on Facebook.

"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."

On-court microphones picked up a sexual slur uttered by Kyrgios who said that Wawrinka's "girlfriend" had been involved with his close friend and fellow Australian Thanasi Kokkinakis.

"Sorry to tell you that mate," Kyrgios is heard to say with his back to Wawrinka at the time.

The ATP said that Kyrgios had been fined the maximum on-site amount of US$10,000 but added they were "reviewing the matter and additional penalties may be forthcoming".

Kyrgios has already pocketed almost US$750,000 in prize money this year and US$1.55 million in his young career.

World No. 1 Novak Djokovic said that Kyrgios deserved to be punished.

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

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

French Open champion Wawrinka was furious when he heard what Kyrgios had said.

"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 said in a Twitter statement.

'STOOP SO LOW'

"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. To stoop so low is not only unacceptable but also beyond belief," the Swiss player added.

Kyrgios is due back on court later on Thursday when he takes on John Isner of the United States for a place in the quarter-finals.

Kyrgios' comments were made after the first set on Wednesday which he won 7-6 (10/8).

Wawrinka won the second set 6-4, but withdrew with back problems when he was losing the third set 4-0.

Wawrinka, 30, split from his wife earlier this year and has been linked to Croatian tour player Donna Vekic.

"It's not the first time that he has big problems on court in terms of what he says and in terms of how he acts. I hope the governing body does not stand for this," added Wawrinka.

Kyrgios, whose Twitter motto is "Play Your Own Rules", has been the target of criticism from the tennis establishment several times for his on-court behaviour.

At Wimbledon, he drew jeers from the public as he apparently did not bother to compete for a game in the second set of his fourth-round defeat by Richard Gasquet.

He also had problems with chair umpires on occasion during the fortnight.

Kyrgios defended his action with Wawrinka after the match.

"He was getting a bit lippy with me. Kind of in the heat of the moment, I don't know, I just said it," said the Australian.

Wawrinka's coach, Magnus Norman, said in a tweet: "That was really really low Nick Kyrgios. Hope for u that u have people around that will teach u a thing or 2 about life tonight. Very bad??"