PARIS: Roger Federer was at the top of his game in London last week, sweeping past opponents as if his age were not an issue. A back injury over the weekend changed everything, casting serious doubts over his chances of winning the Davis Cup, or even playing against France in the final.

The 17-time Grand Slam champion picked up the injury in a hard-fought win over Swiss teammate Stan Wawrinka at the ATP Finals, forcing him to retire ahead of the title match against Novak Djokovic.

One statistic speaks volumes about the seriousness of the 33-year-old Federer's injury: His walkover at the season finale marked only the third time in his career that he withdrew before a match, each time due to back problems.

As if Federer's injury concerns were not enough, the Swiss are also worried about Wawrinka's morale as the country bids for its first Davis Cup victory. After wasting four match points against Federer in London, the Australian Open champion said he “can either be destroyed or bounce back” from such a tough loss.

Following the match, two men are also said to have been involved in a heated spat after Wawrinka complained about noise coming from Federer's box in between serves toward the end of the third set.

Federer defused the tensions by posting a picture on Twitter of a united Switzerland team, with Wawrinka making bunny ears behind his teammate, and both players insisted on their friendship on Tuesday.

“We had a conversation after the match. Everything's totally relaxed about the situation. We're old enough,” Federer said. “I just wanted to see if there were any hard feelings because it was probably one of the loudest moments of the match. Clearly a lot of noise. Like I said, there is no hard feeling whatsoever. We're having a good time here. We are friends, not enemies.”

Federer traveled to the northern French city of Lille by private jet on Monday, hoping to recover in time before the best-of-five series beginning Friday with two singles matches, followed by doubles on Saturday and reverse singles on Sunday.

“It was definitely not good enough to practice yesterday,” Federer told reporters Tuesday. “I wish progress would be faster, but we're trying hard. We're heading in that direction. I feel it's definitely a little bit better than it was on Saturday night and Sunday and also Monday. Baby steps, I guess. I'm hopeful.”

Federer had been in tremendous form this season, playing a more offensive game and bagging five titles while staying injury-free after back problems damaged his 2013 campaign. Chasing the only major title still eluding him, Federer also showed his commitment to the Davis Cup this year, joining the Swiss team immediately after his loss in the US Open semifinals to take part to their semifinal against Italy in September.

For years, Federer did not regard the Davis Cup as a main priority. The emergence of Wawrinka as a top player changed the equation and convinced him they had a good chance of winning the event. But without Federer, the Swiss have won just one of the 10 ties they have played in the World Group since his debut in the competition in 1999.

If Federer can't play, Swiss captain Severin Luthi will have to do either with the 212th-ranked Marco Chiudinelli or No. 508 Michael Lammer. “For the time being, we are not thinking that Roger Federer will not play on Friday,” France captain Arnaud Clement said. “We have been preparing for 10 days, and we are prepared to play the Swiss team with Federer and Wawrinka. But what we are going to play is not Federer's team, it is the Swiss team. They have good players.”

Unlike its opponents, France is at full strength with Jo-Wilfried Tsonga, Gael Monfils, Julien Benneteau and Richard Gasquet having traveled to Lille after a training camp in Bordeaux. Clement also summoned Gilles Simon as a reserve player.

Both Tsonga and Gasquet, who won their singles matches and teamed up in doubles in the victory over the Czech Republic in the semifinals, said they are not getting distracted by the Swiss team's struggles. France is looking to win the trophy for the 10th time.

“I think for this weekend, the important thing for us is to stay focused on ourselves, on our team,” Tsonga said. “What we need is to have a perfect performance. What we can do is just play tennis. Anything else is not really our business.”