Andy Murray up for the fight after hitting back to beat Kei Nishikori

The world number one stretched his winning streak to 21 matches as he maintained his perfect start in a tough round-robin pool courtesy of a resilient 6-7 6-4 6-4 success over the Japanese at the O2 Arena.

It was a truly gruelling affair that lasted a lung-busting three hours and 20 minutes with Murray's struggles typified by a low first-serve percentage. He will need to remedy that if he is to go the distance and out-perform Novak Djokovic at the competition to finish 2016 on top of the men's rankings.

It is the first time since the Scot's debut at the tournament back in 2008 that he has triumphed in his opening two matches.

"I wasn't able to dictate many of the points," said Murray. "More so in the third set. But not in the first couple. I was having to run, fight, get as many balls back as I could.

"I eventually got over the line. These are the sort of matches you work so hard for."

With two wins from two, Murray would have been assured a semi-final berth had Marin Cilic beaten Stan Wawrinka in the later group match but the Swiss recovered from his drubbing by Nishikori to win 7-6(3) 7-6(3).

Murray will now face US Open champion Wawrinka on Friday knowing anything but a straight-sets defeat would suffice.

"Hopefully there will be another three days to this season and I'll do my best to get through them."

Nishikori had won just two of his nine meetings with the Brit going into the match, but he did beat Murray in their last showdown in a five-set epic at the US Open in September.

It had clearly given him a blueprint for success and his confidence was evident as he attacked the Murray second serve. The gameplan looked to be working in the early stages as the Scot battled hard to save break points in three of his first six service games.

In contrast, Murray didn't create an opportunity on the Nishikori serve in over an hour of play until the 12th game where he conjured up a set point. It would have been harsh on the Japanese, who had edged the opener, and he deservedly hit back to set up what turned out to be an exhilarating tie break lasting over 15 minutes.

Nishikori had triple set point at 6-3, with two on his own serve, but then had to deny Murray on two other occasions before finally taking an opening set lasting one hour and 26 minutes 11-9 on his fifth opportunity of the breaker.

Murray immediately responded with the first break of the contest and it remained with serve until Nishikori pulled level at 4-4.

Murray hit back by winning one of the best games of the match from deuce, stealing a 21-shot rally along the way. The Scot then saved two break points before serving out the set on his third chance to move level.

In the third game of the decider the world number one twice got lucky with a net cord to gain the advantage and then held off the threat of a break back on two occasions to consolidate.

It was a key period in the match as Murray clocked up five games in a row in a purple patch that also saw his opponent double fault to gift him the double break.

It proved to be significant breathing space as Nishikori dug in to take the next three on the spin before Murray served it out at the second time of asking to conclude what was the longest match at this tournament since 1991.

Murray will aim look to finish the round-robin stage with a 100 per cent record against the Swiss on Friday to ensure he tops the group and avoids a potential semi-final showdown with Djokovic.