Novak Djokovic of Serbia kisses the trophy after Roger Federer of Switzerland forfeited due to injury in the men's singles final at the ATP World Tour Finals at the O2 in London, Nov 16, 2014. [Photo/Agencies]

LONDON - World No 1 Novak Djokovic won his fourth title at the ATP World Tour Finals in London on Sunday without playing any tennis after six-time champion Roger Federer withdrew due to a back injury.

The Serbian also became the first player to win three straight season finale titles since Ivan Lendl from 1985-87.

Djokovic, who also won at Shanghai in 2008, extended his winning streak at the tournament to 14 matches.

"We always give 100 per cent," said Djokovic holding the trophy. "I'm sure that if he could've played, he would've done. So I wish him the best in his recovery.

"I'm not one of those players that celebrate these particular wins, but I have to reflect on the whole season. This trophy is definitely the crown for all the achievements that I had this year. I owe the credit to my team, to all the people who supported me all the way through, and plenty of joy. I'm very happy to be able to stand here and finish with this trophy."

Djokovic, who has finished year-end No. 1 in the ATP Rankings for the third time (also 2011-12), completed his season with a 61-8 match record. He has won seven titles from 11 finals.

Federer, 33, made the announcement of withdrawal to fans following the doubles trophy ceremony.

"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, 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.

"I wanted to come out personally and excuse myself. 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."

It is only the third time in Federer' s career that he has been forced to withdraw, each time due to a back injury. This is the first walkover in a final in the tournament's 45-year history.