Roger Federer's hopes of being fully fit to play for Switzerland in Friday's opening two rubbers of the Davis Cup final against France dimmed even further when he again missed training on Wednesday.

The 17-time major winner, who has never won the Davis Cup, injured his back playing in the semi-finals of the World Tour Finals in London on Saturday.

He won that gruelling three-set marathon against Swiss team-mate Stan Wawrinka after saving four match points, but paid a hefty price for his efforts and was unable the following day to take on Novak Djokovic in the final.

Since arriving in Lille on Monday he has been unable to practise on the indoor claycourt the French have chosen for the final.

He did have an early afternoon slot available on Wednesday but did not appear, leaving Wawrinka and the other Swiss players to practise by themselves.

Tuesday evening Federer said he was unable to ascertain just what his chances were of being able to play on Friday and was taking it day by day.

"I'm making some progress. But I know I don't have a month ahead of me to get better. I need to get better quickly. I'm trying whatever I can," he said.

Federer will have one more chance to practise on the final court later Wednesday ahead of Thursday's draw.

But already there is speculation that the Swiss team may decide not to take the risk and rest him up for Sunday and the reverse singles rubbers.

The problem for the Swiss is that although Wawrinka, at fourth in the world, is comfortably ranked above all the French players, after him the fall-off in the Swiss team is steep with Marco Chiudinelli 212th and Michael Lammer 508th.

Without Federer, their chances of winning the Davis Cup for the first time look slim, with the French boasting a strength-in-depth line-up of Jo-Wilfried Tsonga, Gael Monfils, Richard Gasquet and Julien Benneteau.