Nick Kyrgios has blamed his walk-off in the Shanghai Masters on a stomach bug after quitting his first-round match with Steve Johnson after the first set.
The Australian lost the tie-break 7-5 soon after receiving a warning for smashing two balls out of court in frustration at a line call.
Kyrgios was heard telling his team that he would call it quits if he lost the tie-break and went on to halt the match moments later to a chorus of boos from the crowd.
Following Tuesday's incident, the 22-year-old wrote 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."
The incident came three days after Kyrgios suffered a point penalty during his China Open final defeat to Rafael Nadal due to an ongoing argument with the umpire.