WTA Tour chairman Stacey Allaster and women’s tennis legend Martina Navratilova demanded tougher punishment for Nick Kyrgios in the wake of his sex slur during a match.

Kyrgios was fined $US12,500 ($A17,000) by the ATP Tour on Thursday as it launched an investigation into the 20-year-old’s remark to French Open champion Stan Wawrinka in a second-round match Wednesday at the ATP Rogers Cup in Montreal.

“The statement made by Nick Kyrgios is crude and unacceptable,” Allaster said in a statement.

“I commend the ATP on assessing the on-site maximum fine and urge there to be further sanctions in accordance with their process.”

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

On-court microphones picked up the Australian’s scurrilous sexual remark about Wawrinka’s girlfriend during a changeover in their match won by Kyrgios when Wawrinka retired with back pain trailing 4-0 in the third set.

Kyrgios said that the woman had been involved with his close friend Thanasi Kokkinakis.

“Sorry to tell you that mate,” Kyrgios is heard to say with his back turned to Wawrinka.

Kyrgios said he has apologised to Wawrinka and hopes to move on but 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.”

Navratilova tweeted that she agreed with that notion, adding: “I think there needs to be more than just a fine. There is no place for that kind of behaviour, that is clear.”

World No.1 Serena Williams, asked about the remarks after a match late Thursday in Toronto, said she did not want to get involved in the controversy by expressing any opinion.

“I think a lot of people saw, but as I said at Wimbledon, I’m no longer involved in controversy, so I don’t want to be involved in any controversies,” Williams said.

Kyrgios was knocked out of the Montreal tournament on Thursday by John Isner of the United States.

He was loudly booed both on his arrival and exit from the court.