Nick Kyrgios caused controversy at the Shanghai Masters for the second year in a row when he retired after the first set of his match against Steve Johnson in an apparent protest at the officiating.
The Australian served a ban from the ATP Tour last year after showing a lack of effort in a second-round match against Mischa Zverev and arguing with the umpire and fans.
Kyrgios could find himself in hot water again if he does not have a legitimate explanation for his decision to retire after losing the first set of his opening-round match 7-5 on a tie-break.
The 22-year-old, who lost in the final of the China Open to Rafael Nadal on Sunday, looked distracted and grumpy from the start of the match.
He received a warning for smashing two balls angrily out of court in frustration at a line call but held his serve to force a tie-break.
Kyrgios won the first three points but Johnson pulled back to level and umpire Fergus Murphy then gave Kyrgios a point penalty for an audible obscenity.
The 13th seed was heard to say he would quit if he lost the tie-break and, after Johnson served an ace on his first set point, Kyrgios walked over to shake hands.
Murphy asked Kyrgios if he needed medical help but, with boos ringing out, the Australian put his rackets in his bag and walked off court.