Novak Djokovic squeezed past Britain's Andy Murray in the latest chapter of their evolving rivalry on Wednesday but ended the day still uncertain of place in the last four at the ATP World Tour Finals after Tomas Berdych overpowered Jo-Wilfried Tsonga.
Once again there was barely a cigarette paper between the two 25-year-olds as Djokovic chiselled out a 4-6 6-3 7-5 victory to take command of Group A at the season-ending showdown.
Victory for Tsonga later against Berdych would have guaranteed Djokovic a top-two finish in the group and progress to the semis but Berdych's 7-5 3-6 6-1 victory means all four players, even winless Tsonga, could still qualify.
Djokovic, who has two wins from two matches, faces Berdych on Friday when victory would guarantee top spot in the group. Murray, who like Berdych has one win, plays Tsonga knowing a straight-sets victory would clinch a semi-final spot.
"I don't like mathematics," fifth seed Berdych told reporters when asked about the various scenarios that could unfold when the group concludes on Friday.
"Tennis is not like football when you can play for a draw in the match. It's one or the other, that's it."
The last thing on the minds of Djokovic and Murray earlier were sets percentages and games countbacks as they set about each other with the sort of relish that marks out their duels as "must see" matches.
Apart from the opening set when world number three Murray could do no wrong, the quality did not quite reach the heights of this year's Australian Open semi-final or the U.S. Open final, but it was riveting fare all the same.
Some of the rallies resembled high-speed chess as both players probed for non-existent weaknesses.
Single points often became mini epics in their own right as first Murray held sway, then Djokovic turned things around before the Serb held off a late Murray fightback to seal victory in two hours 34 minutes.
It was Djokovic's fourth victory over Murray this year with the other three of their seven clashes going the way of the Scot, most notably an epic five-setter at the U.S. Open when Murray became the first British man in 76 years to win a grand slam title.
"Another great match, another great performance from both of us," Djokovic, who is guaranteed to end a second successive year on top of the ATP rankings, told reporters.
"I didn't expect anything other than a tough match that went down the wire and was decided in the last point."
"It was really important obviously for me to win this match today, as it was for Andy. But I will not try to depend on some other players' scores and matches.
"I will try to focus and win every match that I play," Djokovic, who will only need to win a set against Berdych on Friday, added.
A miserly Murray dropped only three points on his serve in the opening set, offering Djokovic no way into the match.
However, Djokovic refused to panic, even when facing potential critical break point at 1-1 in the second set when Murray was still bossing the rallies.
He produced a moment of magic, bravely taking on a drive volley and landing the ball flush on the intersection between baseline and sideline.
Djokovic then broke Murray for the first time in the next game when his opponent missed a volley and the momentum shifted his way as the almost inevitable decider loomed.
A terrible service game from Murray gifted Djokovic a break of serve in the third set and the Scot had to scramble to avoid a double break before launching a spirited fightback.
When Murray levelled at 4-4 he looked favourite to win but a lapse at 5-5 allowed Djokovic the chance to serve for the match and he seized it, battling back from 15-40 to seal victory.
"The last two minutes of the match probably is what decided it," Murray, who reached the semis at the 02 in 2012, said.
"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."