Australian tennis bad boy Nick Kyrgios has delivered a stunning piece of on-court banter to world number five Stanislas Wawrinka during an ATP Rogers Cup match in Montreal.

During the match, Kyrgios walked towards his Swiss opponent and delivered a line of personal trash-talk that was picked up by television cameras.

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The 20-year-old Kyrgios, who regularly courted controversy at Wimbledon last month, taunted Wawrinka in a comment that mentioned fellow Australian youngster Thanasi Kokkinakis.

“Kokkinakis banged your girlfriend, sorry to tell you that,” Kyrgios said.

Watch the video below to hear Kyrgios’ incredible comment

Kokkinakis, 19, is a close friend of Kyrgios and the pair have often played doubles together in the past.

The 30-year-old Wawrinka – who split up with his wife in April – is reportedly dating Croatian tennis player Donna Vekic, who is 19.

Kyrgios won the match after Wawrinka retired hurt, trailing 0-4 in the third set. Wawrinka won the first set 7-6 and Kyrgios took the second 6-3.

After the match, Kyrgios said he was unsure why he made the comment: “He was getting a bit lippy with me.

“[It was] kind of in the heat of the moment. I don’t know. I just said it.”

The Aussie will face John Isner in the next round.

Earlier this week, it was revealed that Kyrgios will be mentored by Aussie veteran Lleyton Hewitt.