Andy Murray hopes to recover from his win against Kei Nishikori in time to face Stan Wawrinka at the ATP World Tour Finals on Friday.
The 29 year old Briton needed a tournament-record three hours and 20 minutes to beat Nishikori on Wednesday.
He returns to London’s O2 Arena 48 hours later knowing he must see off Wawrinka to ensure a semi-final place.
Murray will play the Swiss third seed at 14:00 GMT on Friday.
Nishikori takes on Marin Cilic in the evening session at 20:00, with the Japanese player also able to claim one of the two semi-final places from the John McEnroe Group.
Murray needs to qualify for the last four to maintain his hopes of ending 2016 as the world number one, ahead of Novak Djokovic.
The only way Murray can be eliminated after winning his opening two matches is with a straight-set defeat against Wawrinka, followed by a Nishikori win over Cilic.
To win the group and avoid playing Djokovic in the semi-finals, Murray must either win, or lose in three sets and hope Nishikori beats Cilic in three sets.
“Obviously it’s important to win matches to give yourself the best chance to go through, and 200 points for each match here is quite a lot as well,” said Murray.
Asked if he was optimistic he could recover from Wednesday’s lengthy match in time to challenge Wawrinka, the Scot told BBC Sport: “I haven’t thought about that.
“I’m just getting up each day and trying to give the best of what I have on that day.
“I’ll just try to use a good rest day to recover. Hopefully I won’t be too stiff and sore come Friday, but it’s not ideal to play matches of that length at this stage of the season.”
Wawrinka played poorly in a swift defeat against Nishikori on Monday but improved dramatically to beat Cilic and resurrect his hopes on Wednesday.
The Swiss, 31, lost to Murray in their only meeting this year at the French Open but won last year’s group-stage contest at the O2 Arena.
And the US Open champion has no qualms about potentially knocking out the home favourite.
“I don’t care at all,” said Wawrinka. “All I know is if I want to have a chance to qualify, I need to win. That’s all I know. That’s all I’m trying to do.
“I’m going to try to play my best tennis, to beat the world number one.”