Nick Kyrgios of Australia
Sweat drips from the chin of Nick Kyrgios of Australia as he prepares to serve against Andy Murray of Britain during their men's singles quarter-final match at the Australian Open 2015 tennis tournament in Melbourne January 27, 2015. Reuters/Thomas Peter

Australian professional tennis player Nick Kyrgios has tweeted an apology for passing comments on his Swiss opponent Stan Wawrinka’s girlfriend on-court and has been fined US$10,000 [AU$13,580] by the Association of Tennis Professionals.

In the tweet, Kyrgios mentioned that he was apologising for telling Wawrinka that his girlfriend had slept with fellow player Thanasi Kokkinakis, during their Montreal match. He also took to Facebook to apologise.

"My comments were made in the heat of the moment and were unacceptable on many levels," his post on Facebook. Kyrgios further mentioned that he had issued a private apology and highly regretted the events that had transpired.

Wawrinka, who is reportedly dating the Croatian player Donna Vekic after his split from his wife Ilham Vuilloud, withdrew himself from Wednesday's Rogers Cup match in the third set due to back pain, ignoring the insult.

Although the players shook hands in the end, the anger in Wawrinka’s face could hardly be ignored.

Kyrgios while walking back to his seat after losing the first-set tiebreak was heard saying: " Kokkinakis banged your girlfriend, sorry to tell you that mate ." It is not specifically mentioned if Kyrgios was referring to Vekic. The news of his apology came just after ATP announced that he was fined for his crude comments.

Video via The Guardian

The ATP said on Thursday that an action was taken against 20-year-old Kyrgios for passing such comments while the match was on. The comments were picked up by a courtside microphone.

"The ATP has announced that Nick Kyrgios has been fined the maximum on-site amount of AU$13,580 for an insulting comment he made to Stan Wawrinka during their second round match at the Rogers Cup in Montreal, Canada," an spokeswoman said in a statement.

The statement added the ATP is still reviewing the matter, and as a result more penalties might be added.

Meanwhile, 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.

He asserted that Kyrgios shouldn’t be excused for what he said, believing that this punishment would teach him a lesson for life. However, Djokovic also praised him for his work and said that they never had any personal problems.

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

Meanwhile, Wawrinka also tweeted that the behaviour of his fellow colleague was unacceptable and said that he wouldn’t even mention such things to his rivals. “To stop (sic) so low is not only unacceptable but also beyond belief," Wawrinka wrote on Twitter .

Back in 2014, Kokkinakis and Vekic were linked during their match doubles at the Australian open. In a post-match statement, Wawrinka also mentioned confronting Kyrgios in the locker room post-match.

The ATP has released a statement saying that further details of the fine will be revealed only after Kyrgios has been notified.