Nick Kyrgios was warned for bad language and docked a point for unsportsmanlike conduct after the Australian threw his racquet into the stands during Friday's 6-3 6-4 defeat by Borna Coric in the Murray River Open at Melbourne Park.

Kyrgios' problems began early on Friday as he struggled with a left knee issue and needed treatment midway through the match at the Australian Open warm-up event.

"I can't serve without pain. When I land it feels unstable," media quoted Kyrgios as telling a trainer between sets.

"I'll probably just play one more game. I'm serving at like 170 (kph) instead of 220."

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The 25-year-old carried on and was warned by the chair umpire for swearing at 4-4 in the second set. After his serve was broken, he smashed his racquet and threw it into the empty stands.

On Wednesday, Kyrgios had needed assurances from a tournament supervisor that he would not be fined by the ATP for his behaviour in the last round against Harry Bourchier before agreeing to play on.

Win for Wawrinka

Former Australian Open champion Stan Wawrinka was made to work hard for a win by Alex Bolt before prevailing 6-4 4-6 7-6(5).

The Swiss top seed then pulled out of his quarter-final match against Jeremy Chardy later in the day due to fatigue.

All quarter-final matches at the event were scheduled for Friday after action was suspended a day earlier when more than 500 players and officials were tested for COVID-19 after a quarantine hotel worker in Melbourne contracted the virus.