Novak Djokovic fought back from a set down to earn his fourth win of 2012 over Andy Murray and move closer to the ATP World Tour Finals' last four.
The Serb won 4-6 6-3 7-5 at London's O2 Arena, keeping the world number one top of Group A in the round-robin stage.
Tomas Berdych beat Jo-Wilfried Tsonga 7-5 3-6 6-1 in the late match to leave all four players still in the running for the two semi-final places.
Murray's fate will be decided in Friday's final group match with Tsonga.
A straight-sets win would guarantee him a place in the last four but any other result could leave him vulnerable to elimination.
More than 17,000 spectators in London witnessed another engrossing contest of fluctuating fortunes between Murray and Djokovic, with the Serb grabbing the ascendancy in the final three games to win in two hours and 34 minutes.
The result was in doubt until the very final moment as Djokovic had to save two break points before serving out, but Murray was made to pay for a few sloppy games.
"In the last two minutes of the match is probably what decided it," said Murray. "He broke from 15-40 and then I had 15-40 next game and didn't break, so that was the moment that decided the match."
Djokovic said: "I don't think I've played bad in the first set. It was him playing really well, serving extremely well. He lost only a couple of points on his first serve throughout the whole set, so that says enough about his quality.
"And then, you know, he made some unforced errors. He allowed me to get back to the match. Then I think it was quite even up to the last point."
It was the seventh time the pair had met this year, with three wins apiece before Wednesday's clash, and Murray appeared well set to make it four when he broke serve straight away with a forehand winner.
The Scot was dominant early on, dropping just one point in his opening four service games, and after some shaky moments in recent matches he closed out the set with an impressive love hold.
Djokovic was struggling with his backhand, failing to build pressure as he threw in regular errors, and Murray had a glimpse of the finish line with a break point at 1-1 in the second set.
With defeat looming, the world number one escaped with a shot of pure class. He took a floating ball out of the air with a fierce drive-volley that landed smack on the junction of baseline and tramline.
It was a stunning stroke, prompting Murray to challenge in disbelief - but Hawkeye confirmed it was good.
Djokovic gathered strength game by game from that moment, while Murray's focus wavered. When the Briton faced a key moment at break point down in game six, he unexpectedly chose to serve-volley.
Had it come off, Murray might have been hailed for a masterful change of tactics, but as it was he failed to control the Djokovic return and volleyed long over the baseline.
In keeping with the other epic battles between the pair in 2012, it came down to a final set, and it was Djokovic who made the first move off the back off a slack Murray service game at 1-1.
There was plenty of time left for the world number three to gather himself for another tilt at the top seed, and from 4-2 down he reeled off three straight games and moved to within two points of victory at 5-4.
However, Djokovic has become the hardest man in tennis to finish off and he served his way out of trouble before breaking for the third time in the match with a heavy forehand return.
One more escape effort was required at 15-40 when he served for the match, but this time there was no reprieve for Murray and he must now gather himself for a battle with Tsonga on Friday.
"You have to play the match and try and win regardless of the situation either of us are in," Murray said of Friday's match.