Andy Murray hopes to recover from his epic win over Kei Nishikori in time to face Stan Wawrinka at the ATP World Tour Finals on Friday.
The Briton, 29, needed a tournament-record three hours and 20 minutes to see off Nishikori on Wednesday.
He returns to London’s O2 Arena 48 hours later knowing he must take a set off Wawrinka to ensure a semi-final place.
Murray will play the Swiss third seed at 14:00 GMT on Friday.
The match will be live on BBC Two, online and Radio 5 live sports extra.
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 by 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.