Shanghai - Australian firebrand Nick Kyrgios apologised and said he had been unwell after he stormed off court to loud boos midway through his opening match at the Shanghai Masters on Tuesday.
The 22-year-old issued a statement following his abrupt exit after losing the first set 7/5 on a tie break to the American Steve Johnson, having been docked a point for the second match running because of his conduct.
The ATP Tour said it had launched an investigation and Kyrgios, similarly hit with a point penalty in losing the China Open final to Rafael Nadal on Sunday, said he was "gutted" to retire against Johnson.
A member of Kyrgios's coaching team told AFP in Shanghai as the player exited the arena that the world-ranked 21 had injured his shoulder.
Hours later Kyrgios, whose undoubted talent has often been betrayed by his quickfire temper, made a statement on Twitter apologising.
"I've been battling a stomach bug for the past 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," he wrote.
"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 the past 24 hours."
The Australian 13th seed said he will make a decision on Wednesday as to whether he takes part in his doubles match the same day with Frenchman Lucas Pouille.
Kyrgios hit a lowpoint in Shanghai a year ago when he similarly left the court to jeers, having rowed with the crowd and appeared to give up in his match against Germany's Mischa Zverev. Kyrgios was consequently suspended.
The temperamental Kyrgios did look in discomfort with his shoulder early on in the match against the unseeded Johnson.
But he seemed untroubled by the injury when they entered the first-set tie break, but like in the final in Beijing, his temper soon got the better of him, after he was angered by the unsettled crowd and the umpire.
He began Tuesday's match on his best behaviour but showed a first flash of irritation towards the end of the set, blasting a ball out of the court after he disagreed with a call.
Kyrgios and Johnson were both comfortable on their serves and they went into the tie break with signs Kyrgios was beginning to lose his cool.
He went 3-0 up in the tie break but as Johnson pegged him back he was irked by spectators in the close-knit arena getting up between points.
"What's happening here?" he asked, before muttering: "Exactly why I shouldn't come here."
Kyrgios appeared to blame the umpire for failing to get the spectators in line and his afternoon spiralled fast when he received a point penalty.
"What is he doing? Is this normal?" Kyrgios asked somebody in the official area of the arena, where spectators and players are in close proximity.
"Poor officiating again," Kyrgios then complained, and added: "Sick of this place."
Johnson wrapped up the set and Kyrgios decided he had had enough, shaking the hands of his opponent and the umpire -- though not looking him in the eye -- and packing his bag and leaving.
"I'm gutted to be honest as I was keen to keep the good momentum that I built up in Beijing going and finish the year strong," Kyrgios added in his statement.
"I'll do what I can to make sure I do."