The Briton and reigning Wimbledon champion lost the first set 11-9 in a tie break against the Japanese star, who knocked him out of this year's US Open.

But Murray battled back to win the second and third sets 6-4, 6-4 in a match lasting three hours and 20 minutes.

It was the longest match of the annual season-ending event since it moved to London in 2009. 

The 29-year-old has not yet qualified for the semi-finals but is in pole position to do so.

It was also Murray's 21st victory in succession as the Briton seeks to match the result of Serbian rival Novak Djokovic in order to remain ahead of him in the rankings.

"Who knows what's going to happen the next few days?" Murray said.

"I'll concentrate on trying to win my own matches, get through as many as I can and make it as tough as possible for Novak to jump me."

Third seed Wawrinka recovered from his opening loss to win 7-6, 7-6 over seventh seed Cilic in a match completed in less than two hours.