Being able to come out on top after a physically draining encounter with Kei Nishikori makes rigorous preparation and exhaustive attention to detail worthwhile, according to Andy Murray.

The world number one recovered from losing the opening set to defeat the Japanese in three hours and 23 minutes at the O2 Arena on Tuesday - the longest best-of-three-sets match in the ATP World Tour Finals since 1981, when records began.

Murray, like fellow heavyweight Novak Djokovic, is renowned for his superhuman levels of fitness.

And they came to the fore again to account for Nishikori in London, much to the satisfaction of the Wimbledon champion.

"That's what you work for, for these moments, big matches, places like this, with [an] amazing atmosphere," Murray said in an on-court interview after his victory.

"I just fought really hard, Kei was making me run a lot, he was dictating a bunch of the points, but I was starting to get some free points on my serve and I was making him work hard on his service games. [I] managed to get enough breaks to win.

"Me and Kei have played three times this year, we played five hours in Davis Cup, four and a half hours at the US Open ... we've played a lot of long ones this year."

Murray, who is on course to top Group John McEnroe and book a semi-final against the runner-up of Group Ivan Lendl - already won by Djokovic - expects to be suffering for his exertions.

"I feel OK right now but it's normally kind of the following day when you feel stiff and sore maybe later on that evening," he said. 

"There's hopefully three days left of the season and I'll give my best to get through as many matches as I can."