Federer's aching back could derail his Davis Cup dream

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."