The two best players in the world, Andy Murray and Novak Djokovic, are not exactly having an easy time of it in the last tournament of the season. They head for an almost nailed-on weekend showdown, the 35th of their careers, unbeaten and resolute but looking just a little weary, with maybe a twist or two to come.

The Scot, buoyed by an O 2 crowd of 17,000 fans as unequivocally behind him as ever they were for Tim Henman, saved nine of 11 break points over three hours and 20 minutes – the longest match in the eight years of the ATP World Tour Finals at this venue – to get past Kei Nishikori 6-7 (11-9), 6-4, 6-4.

Andy Murray battles back to beat Kei Nishikori – ATP World Tour Finals! Read more

There was a late-night complication when Stan Wawrinka beat Marin Cilic 7-6 (7-3), 7-6 (7-3), putting the Croat out of the tournament and leaving the Swiss lurking as a dangerous wildcard. If Nishikori beats Cilic in their final round-robin match, Murray will need a strong finish against Wawrinka on Friday to guarantee avoiding Djokovic in the semi-finals. He would still go through if he wins only a set, but in second place – and that is not where the world’s new No1 would ideally like to be. Disaster is not imminent. However, as history tells us, this tournament is a crazy beast.

Murray said: “It could come down to a match between me and Novak [in the race for No1 in the world]. Who knows what’s going to happen the next few days. From my side, I will concentrate on trying to win my own matches, get through as many as I can, make it as tough as possible for Novak to jump me.”

Getting through the second match of his group was a minor marathon, more white-knuckle edgy than the Serb’s stop-go progress through two wins in the round-robin stage on the other side of the draw. It says much about the physicality of the Tour that its best champions are close to exhaustion just as they prepare for one last round of the fight.

Still, they remain on track to meet in Sunday’s final and slug it out for Murray’s ranking. That is where the core fascination of the tournament has resided since the first match, when Djokovic came from a set down to beat Dominic Thiem.

Djokovic, who surrendered the top spot in Paris, where Murray went on to win his fourth title in a row, briefly moved back in front of him in the rankings on Tuesday night when it took him two tight sets to blunt the power-serving of Milos Raonic.Murray had not won two matches in a row in this end-of-season tournament since his first appearance in Shanghai during 2008. But, a semi-finalist three times, in 2008, 2010 and 2012, he is a more mature and self-assured player at 29. And a knackered one.

His second win on the spin here, 21st over the past two months and eighth in 10 matches over Nishikori, was far tougher than his commanding 6-3, 6-2 workout in the first round of his group, against Cilic.

Murray’s coach Ivan Lendl, a five-times winner of this title, joined his year-round assistant, Jamie Delgado, courtside and probably was more impressed with the player’s trademark resilience than his fluency. When Murray belted a regulation forehand into the high section of the net rather than going for the angle on set point, Nishikori held to force the first set tie-break.

It had seemed inevitable from the first ball. Murray was 20-8 in shootouts for the season, Nishikori 16-10, although the Scot was looking the more agitated, constantly chuntering to himself. After giving up three set points he showed extraordinary anticipation to hunt down an impossible get and stick a running backhand winner behind Nishikori, and the arena rose to applaud probably the shot of the tournament.

However, he blew set point, handed two more back to Nishikori and struck his final shot disappointingly wide to conceded what had been a ragged, tense set after an hour and 25 minutes. The tie-break alone had taken 18 of them.

The snap-back arrived quickly, Murray catching Nishikori off-guard at the start of the second; but after two hours of the match his own tired legs contributed to his dropping serve for 4-4. Within minutes Murray broke back, saved two break points and served out the set. Heaving for breath on his third set point he twice faltered in the ball toss – thanks to a boorish interjection from the crowd – but finished the job when Nishikori’s forehand missed the outer edge of the baseline.

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As some suspected it might do from no more than 20 minutes into the match, it all came down to the third set, Murray territory. He had won a draining five-setter against Nishikori in the Davis Cup but lost in five in the quarter-finals of the US Open.

Murray broke early again. Could he get to the line this time without giving the fans a nervous breakdown? Paradoxically the standard rose as tiredness consumed both players. It was a monumentally committed spectacle. Neither held serve with comfort. Both fought for every point, serving or receiving. No ball was too far away to chase. Each success was celebrated as if it were for the final of a slam.

By the time Murray had pulled away to lead 5-1, the crowd, surely, could relax at last... not quite. Nishikori held for 2-5. Just. Murray double-faulted for 5-3. Nishikori scrambled magnificently for 4-5. And Murray held to love to seal it.

It was sport in the raw, at times ugly, and indisputably passionate.