Roger Federer has sensationally withdrawn from the final of the ATP World Tour Finals with a back injury just minutes before taking on Novak Djokovic in London.

The 33 year old, who will bid for a first Davis Cup with Switzerland against France next week, suffered a recurrence of a back injury in the third set of his grueling victory over fellow-countryman Stanislas Wawrinka on Saturday.

The decision sees Djokovic crowned as the season-tour finals champion for a third straight year, which caps a fabulous year for the Serbian which also saw him secure the year-end world No.1 for a third time in four years.

"I am not match fit to play the match," said Federer. "I didn't want it to end this way. I've been getting pain-killers and treatment but I can't compete with Novak. I hope you [the crowd] understand."

The 17-time grand slam champion added on his official Facebook page: "I am sorry to announce that I cannot play the finals tonight vs. Novak. I hurt my back late in the match yesterday against Stan. I am very disappointed and I hope to be feeling better soon.

"It's been an extremely difficult decision because I love playing in London and the ATP World Tour Finals have been an absolute highlight of my career. Unfortunately, my back problem does not allow me to play right now. I hope all tennis fans and those involved in the event will understand."

"Clearly this is not the way [I wanted to win]," Djokovic said. "I feel sorry for Roger. I've known him for a long time and Rafa [Nadal] and Roger have been the biggest, most competitive and the guy will always give 100% and if he had come out he would have so I wish him all the best.

"He has an important week ahead of him and obviously I am disappointed for the crowd. This was meant to be an anticipated match between the No.1 and No.2 in the world. I am sure everyone was excited to see it. This is the first time in history that we've had something like this happen.

"It is hard to talk about this achievement. I don't celebrate this particular win."

The scheduled final will be replaced by an exhibition first-to-eight pro-set between Djokovic and Andy Murray, before the Briton teams up John McEnroe to face Tim Henman and Pat Cash in a legends doubles match.

The walkover, just the third time in Federer's professional career that he has pulled out of a match, throws into major doubt his fitness for next weeks' Davis Cup final in Lille, particularly given the world No.2 history with injuries of that nature.