Nick Kyrgios has blamed a stomach bug for his mid-match retirement at the Shanghai Masters.
The Australian caused controversy at the tournament for the second straight year by walking off after losing the first set of his opening round match against Steve Johnson.
Kyrgios 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 looked distracted and grumpy from the start against Johnson and, after being given a point penalty for an audible obscenity midway through the tie-break, was heard to say he would quit if he lost the tie-break.
Johnson served it out 7-5 and Kyrgios promptly walked to the net to shake hands, to the bemusement of his opponent and umpire Fergus Murphy.
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.
Kyrgios had grumbled throughout about Murphy and the officiating and, with nothing obvious ailing him and no medical time-outs taken, it appeared frustration had got the better of him.
But the 22-year-old blamed illness in explaining his sudden departure, saying on Twitter: " Guys I want to apologise to the fans in Shanghai and those that watched around the world on TV today.
"I've been battling a stomach bug for the last 24 hours and I tried to be ready but I was really struggling on the court today, which I think was pretty evident from the first point.
"My shoulder started to hurt in the practice today, which didn't help either, and once I lost the first set I was just not strong enough to continue because I've not eaten much for the past 24 hours.
"I'm hoping to be okay for the doubles and will make a decision tomorrow morning, hopefully off of a good night's sleep, which I didn't get last night!
"I'm gutted to be honest as I was keen to keep the good momentum that I built in Beijing going and finish the year strong. I'll do what I can to make sure I do."
Kyrgios arrived in Shanghai on the back of a run to the China Open final on Sunday, which he lost heavily to Rafael Nadal.