Nick Kyrgios brutally sledges Stan Wawrinka during game

Midway through the second set, in a comment picked up by a courtside microphone, Kyrgios told Wawrinka that another teenaged Australian player, Thanasi Kokkinakis, had slept with his girlfriend.

The unpredictable Kyrgios, a good friend of Kokkinakis’, has already endured a tough year, with his on-court behaviour criticised during the Australian Open.

Nick Kyrgios advanced to the 3rd Round in Montreal after Stan Wawrinka retired with a back injury when he was 4-0 down in the 3rd set.

“Kokkinakis banged your girlfriend”.

As for why he said it, well Kyrgios said, “I don’t really know, I just said it”.

“To stop [sic] so low is not only unacceptable but also beyond belief”.

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

Questioned after the match Kyrgios attempted to explain why he made the remark.

Shortly after losing his first set to the higher-seeded player, Kyrgios delivered a shocking verbal forehand smash of sorts to his opponent, a two-time Grand Slam victor.

Neither Wawrinka, nor his coach, Magnus Norman, were impressed and both took to Twitter to voice their displeasure.

‘Kind of in the heat of the moment.

Kyrgios will play American John Isner in the third round of the singles at the US Open lead-up event.

Kyrgios is one of the most colourful and controversial characters on the circuit, with his antics ranging from on-court outbursts at match officials to being accused of giving up half-way through a match at Wimbledon earlier this year.