Roger Federer is uncertain if he will recover from a back injury in time to play for Switzerland in the Davis Cup final, according to Novak Djokovic.

Federer pulled out of his title showdown with Djokovic at the ATP Tour finals on Sunday less than an hour before the scheduled start.

The world No.2 faces a race against time to be fit for the Cup final against France, starting on Friday in Lille.

The 33-year-old aggravated a back problem, which had troubled him at various times this year, during a gruelling three-set semi-final victory over Davis Cup teammate Stan Wawrinka.

Djokovic spoke to Federer later and said he told him it would be a battle to make the first Davis Cup final of his illustrious career.

“I spoke to him and it’s a question mark for the Davis Cup final. He doesn’t know yet,” said Djokovic.

“The stats are saying clearly that he is one of those players that would compete and would always fight if he can. He has had, what, three retirements in his entire career?

“This is probably one of the biggest matches of the year for him. I’m sure if he could have, he would be on the court.”

Federer’s withdrawal made Djokovic the Tour Finals champion for the third successive year – the first player to complete a hat-trick at the prestigious season-ending event since Ivan Lendl in the 1980s, but admitted it was a strange way to end his year.

“During the doubles match, that’s when I found out. I did my warm-up, obviously prepared for the battle, prepared for a great match as it was anticipated, as the people expected it to be,” world No.1 Djokovic said.

“You never like to win, especially … against big rivals, with the retirement. But that’s the way it is.”

During an on-court interview just before the ATP Tour decider was due to begin, six-time champion Federer told the crowd that, unfortunately, he was not match fit to play.

“Clearly I wish it wasn’t this way. You try all year to be ready for the ATP World Tour Finals and I didn’t want to end this way.

“I’m sorry I’m here in a tracksuit. I wish it weren’t so.

“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. I hope you understand.

“I love playing in London and the ATP World Tour Finals have been an absolute highlight of my career,” the 17-time grand slam champion said.

Speculation of a withdrawal mounted when Federer failed to show up for a scheduled practice session on Sunday afternoon.

Fans were already streaming into the 17,500-capacity London venue when the announcement came.

Djokovic agreed to play an exhibition match against Andy Murray to try to at appease a disappointed crowd.