Under-fire tennis bad-boy Nick Krygios could face a more serious sanction for his unsavoury sledge on Stan Wawrinka with the ATP on Thursday announcing an investigation into the remark.

The 20-year-old Kyrgios was fined $US10,000 ($A13,550) for the obscene sexual remark against the Swiss star in their second round match at the Masters, but the Tour said it was also fining him an additional $US2,500 ($A3,400) for another comment made to a ball person.

“Following a review of video from the match the ATP has also issued an additional fine of $2,500 for unsportsmanlike conduct related to a comment he made to a ball person during the match,” the ATP said in a news release.

“In addition, Kyrgios has been served with a ‘notice of investigation’ which begins a process to determine if his actions also constitute a violation of the Player Major Offence provisions.”

The ATP said the probe could result in further sanctions against Kyrgios.

Following a 7-5 6-3 loss to John Isner on Thursday, Kyrgios said he had personally apologised to Wawrinka in the players’ area at Uniprix Stadium and now wants to put the matter behind him, but the ATP seems to have other plans.

“Obviously he saw me in the corridor yesterday,” Kyrgios said of Wawrinka after his loss to Isner. “He came up to me, as you know he would. I wasn’t surprised.

“I told him I was sorry. Obviously he was angry, so I just had to deal with it. Hopefully we can just put it behind. It’s all cleared now. I apologised in public and privately as well.”

On-court microphones picked up Kyrgios linking the reigning French Open champion with teenage Australian player, Thanasi Kokkinakis.

“Kokkinakis banged your girlfriend. Sorry to tell you that mate,” said Kyrgios midway through the second set.

An angry Wawrinka had demanded action by tennis authorities over the “beyond belief” comments.

Wawrinka, who separated from his wife earlier this year and has been linked to Croatian tour player Donna Vekic, said the ATP should take “major action this time”.

Kyrgios hopes the incident won’t dog him as he now travels to Cincinnati for next week’s pre-US Open event.

“I’ve been fined. So everything is sort of put to bed now. I thought we could move on from it,” said the world No.41.

Kyrgios’ insult rocked the sport and the criticism has come from all directions, including world No.1 Novak Djokovic.

Top seed and three-time winner Djokovic racked up his 50th win of 2015 to reach the quarter-finals, scoring his 28th consecutive victory at the Masters level by seeing off Jack Sock of the United States 6-2 6-1.

The Serb then commented on the Kyrgios incident.

“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.

“I understand that every player goes through certain tantrums, emotional ups and downs during the match. In a big fight and at this level, I understand.

“But … there’s no excuse of directing your tantrums to your opponent, especially to somebody that is not even there.”

Djokovic said Kyrgios, fast earning a reputation as the sport’s talented brat, remains one of the potential future stars of tennis.

“He’s young, he’s definitely one of the players that everybody’s looking to,” Djokovic said.

Second seed Andy Murray, who is chasing a third title in Canada, also took his season’s win tally to 50 with a smooth 6-3 6-2 triumph over Luxembourg’s Gilles Muller.

Murray, the 2009 and 2010 champion, will face defending champion Jo-Wilfried Tsonga or Australian Bernard Tomic in the quarter-finals.

The Scot took a softer line to Kyrgios than Djokovic.

“The most important thing is that he learns from what happened yesterday,” Murray said.

“There’s obviously certain things in tennis, in our sport. In other sports that sort of thing happens more often than we imagine, in a lot of team sports especially.

“In tennis there are certain things you shouldn’t do. That was probably one of them.”