Andy Murray will decide on Tuesday whether to pull out of the ATP World Tour Finals after aggravating a groin injury during his loss to David Ferrer.
Murray lost his opening match 6-4 7-5 and later revealed he strained the muscle in his left leg a few days after the Paris Masters.
"I had a problem with my groin," said the British number one. "I'll decide on Tuesday whether or not I keep playing.
"I've been carrying it - it was something that happened in training."
His next match is scheduled for Wednesday, against Tomas Berdych, who was beaten in three sets by Novak Djokovic on Monday night.
If Murray pulls out, he will be replaced by Serbia's Janko Tipsarevic.
"I have to see how it goes, sometimes you've got to do the right thing," he stated. "I didn't know exactly how it was going to feel on the court today.
"I can [technically] decide until one minute before the match on Wednesday but I'll see how I feel tonight and tomorrow and decide."
Murray was beaten by Berdych in the French capital on 11 November and then went straight into training ahead of the season-ending championship at London's O2 Arena.
The Scot mentioned at Saturday's pre-tournament press conference that had been nursing "a few niggles" but there was no suggestion of a serious problem.
However, he clearly struggled to move freely in the early stages against Ferrer and a medical time-out was called at the end of the first set.
Murray received a lengthy massage from the trainer but his discomfort was still evident in the second set as Ferrer came through to triumph in exactly two hours.
The 24-year-old took more than an hour-and-a-half to emerge for his post-match media obligations and looked totally dejected as he delivered news of the injury.
"I haven't been able to do too much training this week," said world number three Murray. "If it wasn't this event or a Grand Slam, I wouldn't have played.
"I just felt it. I've never really had many strains. It's more bony, jointy things that I've had, especially playing on hard courts. It was the first time I'd really felt anything go.
"There's stuff you do any time you have injuries to try to help it - ice it, wearing compression shorts, massage, do an ice bath tonight.
"You can get acupuncture, needles, sometimes those things can help and make it feel better."