Paris (AFP) - Switzerland's hopes of a first ever Davis Cup win lay in the balance on Monday as the team awaited word on just how badly Roger Federer had injured his back ahead of this week's final against France in Lille.

The 17-time Grand Slam winner shocked thousands of fans and his opponent Novak Djokovic in London Sunday evening when he withdrew from the final of the season-ending World Tour Finals. Ironically, the player who helped inflict the back injury on the Swiss great his Davis Cup teammate and close friend Stan Wawrinka.

The two played a thrilling, but punishing semi-final on Saturday evening which Federer, 33, won in three gruelling sets, saving four match points along the way. He told a hushed crowd on court that he had tried everything to be able to play in the prestigious tournament.

What was not clear, however, was just how badly injured he is. Federer has a history of back pain, but until last weekend he had been injury-free throughout a season in which he has played some superb tennis despite failing to add to his Grand Slam title haul.

He is hoping that the back spasms he felt will clear over the next couple of days, allowing him to be able to begin adapting to the indoor claycourt that France as hosts have chosen for the final.

There was some astonishment in the French press over how hard Federer and Wawrinka had gone at it in London, knowing that the Davis Cup final was only a few days away. A Federer withdrawal from the Lille contest would be a huge and potentially lethal body blow to Swiss hopes.

Wawrinka, the world No. 4, is comfortably ranked above all the French players, but after him the fall off in the Swiss team is steep with Marco Chiudinelli 212th and Michael Lammer 508th.

The French, who are seeking a 10th Davis Cup title in all and a first since 2001, has a team boasting of Jo-Wilfried Tsonga, Gael Monfils and Richard Gasquet.