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's 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," local 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."
The 25-year-old, playing his first tournament in nearly a year due to the COVID-19 pandemic, soldiered 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.
In other round of 16 matches on Friday, Frenchman Jeremy Chardy upset sixth-seed Taylor Fritz 6-2, 6-4 while Briton Dan Evans won the opening set of his match with Marcos Giron before rain stopped play.