Novak Djokovic won his third consecutive ATP World Tour Finals title on a walkover after Roger Federer pulled out before the match.

The world No.2 fought for almost three hours to beat countryman Stan Wawrinka in the semi-finals on Saturday night, saving four match points, and did not leave the O2 Arena until the early hours of Sunday.

Fears Federer had back problems again were confirmed when the announcement he had conceded was made after the doubles final.

He came onto court to tell the fans: "Unfortunately I'm not match-fit to play tonight. Clearly I wish it wasn't this way.

"You try all year to be ready for the ATP World Tour Finals … I tried everything I could last night and today - painkillers, rest - until the very end, but I can't compete at this level with Novak.

"In a final like this and at my age, it would be too risky."

On Facebook, Federer wrote: "I hurt my back late in the match yesterday against Stan. I am very disappointed and I hope to be feeling better soon.

Djokovic came onto court to take the trophy and said: "Clearly this is not the way I wanted things to happen. I'm really sorry for Roger not being able to come to the court."

Instead of the final Djokovic played an exhibition game against Andy Murray.