Novak Djokovic won the latest installment of his rivalry with Andy Murray as the world number one fought back to secure a dramatic 4-6, 6-3, 7-5 victory in the ATP Tour Finals at London's O2 Arena on Wednesday.
Djokovic has a 100 percent record after two Group A matches at the season-ending event and will be guaranteed a place in the semi-finals if France's Jo-Wilfried Tsonga defeats Czech fifth seed Tomas Berdych later on Wednesday.
World number three Murray now needs to win his final group match against Tsonga on Friday to avoid an early exit after the US Open champion failed to take advantage of a promising start.
Djokovic and Murray have been on-court rivals and off-court friends since the day they first faced off at an Under-12s tournament in the south of France.
And, with Roger Federer approaching the end of his career and Rafael Nadal hampered by knee problems, their meetings have emerged as the sport's most significant occasions.
In 2012 alone they have clashed seven times, with Djokovic starting the year with victory in the Australian Open semi-finals and Murray getting his revenge with an Olympic last four win and then, even more significantly, a five-set triumph over the Serb to win his first Grand Slam title at the US Open.
Djokovic won their most recent meeting in the Shanghai final after saving five match points and his self-preservation instincts helped him survive another epic showdown lasting two hours and 34 minutes.
With their two Grand Slam meetings this year both timed at just short of five hours, Murray expected another gruelling clash against one of the few players who can match his stamina and defensive skills. He wasn't to be disappointed.
The Scot tried to stamp his authority on the tie as quickly as possible and earned a break point in the first game which he took with a forehand winner on the run.
Murray kept Djokovic at bay with some fine serving and, while he couldn't convert his first set point on the Serb's serve, he sealed the set at the second attempt in the next game.
Djokovic was gradually finding his rhythm however and a lunging volley in the sixth game of the second set secured his first break point, which he converted, celebrating with a wild fist-pump when Murray's volley drifted long, before serving out the set.
There was real power and purpose to Djokovic's game now and he pounced on a rare loose service game from Murray to break for a 2-1 lead in the decider.
But, with the finish line in sight, there was one final twist.
Leading 4-3, Djokovic suddenly lost concentration and Murray, correctly challenging a crucial call on break point, seized his chance to draw level.
That was the cue for the most intense period of the match as both players slugged it out in search of the knockout blow.
It was Djokovic who delivered it, breaking for a 5-4 lead, then saving two break points and finally bringing to a close a decidedly heavyweight bout.