LONDON (AP) ― In a potential blow to Switzerland’s Davis Cup ambitions, Roger Federer pulled out of the ATP Finals less than one hour before his title match against Novak Djokovic on Sunday, handing a third straight title at the year-end event to the top-ranked Serb.

The 17-time Grand Slam champion said he hurt his back in the nearly three-hour semifinal win over Davis Cup teammate Stan Wawrinka on Saturday night in which Federer saved four match points.

“Unfortunately I’m not match fit,” the 33-year-old Swiss told the crowd at the O2 Arena. “I tried everything I could last night, also today: painkillers, treatment, rest, so forth, warm-up, until the very end. But I just can’t compete at this level with Novak. It would be too risky at my age to do this right now and I hope you understand.”

Fans appeared to be supportive with applause for Federer when he spoke. The Swiss is set to play for Switzerland in the Davis Cup final against France, starting Friday. If he recovers in time, Federer will be chasing the only major trophy still eluding him. France hosts the match in the northern city of Lille and has opted for clay, a slow surface that could further thwart Federer’s chances to perform well.

“I don’t think he was calculating and trying to save his body for Davis Cup final,” said Djokovic. “This is probably the biggest match of the season next to the final of a Grand Slam. I spoke to him, it’s a question mark for the Davis Cup final as well.”

It was only the third time in Federer’s career that he withdrew, each time due to a back injury, following walkovers in 2008 at the Paris Masters and in 2012 in Doha.

Following back problems that ruined his 2013 season, Federer enjoyed a superb resurgence this season, losing to Djokovic in an epic Wimbledon final and adding five new titles to his collection.