London, England (SportsNetwork.com) - Roger Federer withdrew from the title match at the ATP World Tour Finals on Sunday because of a back injury, handing world No. 1 Novak Djokovic his third straight championship at the season- ending event.

Federer made the announcement to the crowd after the doubles championship ceremony at the O2 Arena.

"Unfortunately, I'm not match fit to play the match tonight," said Federer on court. "Clearly I wish it wasn't so. I tried all year to be ready for the ATP World Tour Finals, and I didn't want it to end this way. I tried everything I could last night, also today -- painkillers, treatment, rest, so forth, warmup, 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."

Federer went three grueling sets on Saturday in his semifinal against fellow Swiss star Stan Wawrinka, playing just short of three hours and saving four match points to advance to the title match.

The 17-time Grand Slam champion would have been bidding for his seventh title at the season-ending tournament.

"I wanted to come out personally and excuse myself," he continued. "It's been a great week for me. I played some great tennis and I love coming to The O2 and to London, and there's been so many great memories for me here. Congrats of course to Novak, who's played an amazing season, and an amazing tournament here as well. I hope we can play some more great matches, hopefully next year."

Federer still has one more event left on his 2014 schedule with the Davis Cup final against France set for next weekend.

Djokovic, meanwhile, became the first player since Ivan Lendl from 1985-87 to win the season-ending tournament three straight times. He also won the crown in 2008.

It was the first walkover in a final in the tournament's 45-year history.