An injury has forced Stefanos Tsitsipas out his second match at the ATP Finals.

There were doubts over the fitness of the Greek star ahead of the tournament in Turin but he did not appear unduly troubled during an opening loss to Jannik Sinner on Sunday.

However, Tsitsipas barely moved during the third game against Holger Rune and sat down gingerly on his bench trailing 2-1.

He called the trainer but, after a brief chat, he was helped to his feet and shook hands with Rune, leaving the court to a chorus of jeers and whistles from the disappointed crowd.

Speaking at a press conference later, Tsitsipas revealed he has been struggling with a back problem and is withdrawing from the tournament, with Hubert Hurkacz set to take his place.

"My apologies to all the fans and the crowd that came to support me today and watch the match," he said.

"I'm really gutted that I wasn't able to finish the match. It's a shame also not to give that spot to someone else to at least try and do something with it.

"My doctors and the countless visits that I had in the last few days suggested that I play, gave me the green light to go and try it for myself.

"Unfortunately, I felt terrible on the court. I did what I could do in the best possible way to be ready and fit for this match, but it didn't work out for me.

"It kills me not to be able to finish this tournament, the one that I've prepped for for so long."

It counts as a first victory at the tournament for 20-year-old Rune, who was edged out by Novak Djokovic in his opening match.

"It was very unfortunate," said Rune on Amazon Prime Video. "I think we could all see in his first service game that he wasn't serving anywhere near full.

"For sure I knew something was up. We play a long season and it's important to take care of the body. I wish him all the best."