When playing tennis, the ball — and smack talk — must stay within the lines.

Today, men's tennis governing body, the ATP, fined 20-year-old Australian professional tennis player Nick Kyrgios $10,000 for making an insulting on-court remark to his opponent, French Open Champion Stan Wawrinka of Switzerland. Kyrgios was hit with the maximum allowed fine for a verbal offense, but the ATP says is has not ruled out further punishment.

During a second-round Rogers Cup match in Montreal last night, court-side microphones picked up Kyrgios telling Wawrinka that fellow Australian tennis player Thanasi Kokkinakis had slept with a player who is reportedly Wawrinka's girlfriend.

Kyrgios had just lost the first set in a tiebreak when he said to Wawrinka, "Kokkinakis banged your girlfriend. Sorry to tell you that, mate."

Kyrgios was referring to Croat Donna Vekic, 19, who has been linked with Wawrinka, 30. Vekic played mixed doubles with Kokkinakis at the 2014 Australian Open, but the two were not a confirmed couple.

Wawrinka went on to lose the second set 6-3 and retired in the third set, citing back pain.

During the on-court, post-match interview, Kyrgios answered a question regarding his comment, saying, "I thought [Wawrinka] was getting a bit lippy at me, so I don't know, it's just in the moment sort of stuff, but I don't really know. I just said it."

In his post-match press conference, Wawrinka said that he had confronted Kyrgios in the locker room but declined to share what was said between them. The two-time Grand Slam champion did, however, take to Twitter to call for punishment for Kyrgios.

Today, hours before the ATP announced the fine, Kyrgios issued an apology on Facebook:

"I would like to take this opportunity to apologise for the comments I made during the match last night vs Stan Wawrinka. 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."

This isn't the first time Kyrgios has been fined by the ATP. At Wimbledon last month, Kyrgios was fined nearly $9,500 dollars for swearing audibly on the court. His run at the All England Club was also characterized by arguments with chair umpires, smashing of rackets and accusations of tanking when he appeared to give up on returning serve in his match against Richard Gasquet.

After Wimbledon, tennis legend and fellow Australian Rod Laver weighed in on Kyrgios behavior:

"Nick's young and maybe doesn't realize what he is doing sometimes. That's certainly something that he needs to grow out of and he needs to grow out of that sooner rather than later. There's certainly no excuse for swearing. That's just bad behavior, that's ugly."

No word on any Laver comment about Kyrgios' latest transgression, but the message sent by the ATP's fine was loud and clear.

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