Andy Murray fought back to beat Kei Nishikori in a gruelling contest and close in on a semi-final place at the ATP World Tour Finals.
The world number one, 29, lost a tie-break but came through 6-7 (9-11) 6-4 6-4 against the Japanese fifth seed.
At three hours and 20 minutes, it was the longest match at the event since it moved to London in 2009.
Stan Wawrinka beat Marin Cilic 7-6 (7-3) 7-6 (7-3) in the evening match to end the Croat's semi-final hopes.
Murray will face US Open champion Wawrinka in his final round-robin match on Friday.
The only way that Murray can be eliminated is by a straight-sets defeat for himself combined with Nishikori beating Cilic in the other Group John McEnroe match.
After an underwhelming defeat to Nishikori in his O2 opener, Swiss Wawrinka looked close to his best with his forehand proving especially potent.
Murray's success was the first time since his debut in 2008 that the Briton has won his opening two group matches.
The victory, his 21st in succession, had added significance as Murray needs to at least match Novak Djokovic's results in London if he is to stay ahead of the Serb in the year-end rankings.
Djokovic has a 2-0 record in the Ivan Lendl Group, as does the Scot in the John McEnroe Group, having beaten Cilic in his first match.
"It could come down to a match between me and Novak," said Murray.
"Who knows what's going to happen the next few days?
"I'll concentrate on trying to win my own matches, get through as many as I can and make it as tough as possible for Novak to jump me."
Source: Radio Television Brunei