Roger Federer out of ATP World Tour final v Novak Djokovic
Roger Federer has withdrawn from the ATP World Tour final against Novak Djokovic because of a back injury.
The 33-year-old Swiss announced he was "not match-fit" as he apologised in person to the crowd at the O2 Arena.
Federer beat Stan Wawrinka 4-6 7-5 7-6 (8-6) on Saturday in a thrilling semi-final lasting two hours and 48 minutes.
It is Djokovic's fourth victory at the season-ending event and he also becomes the first man to win three titles on the trot since Ivan Lendl in the 1980s.
Federer - who has won the tournament six times - confirmed his withdrawal following the doubles final on Sunday afternoon, won by American brothers Bob and Mike Bryan.
"I'm sorry I'm not fit to play," he said. "I hope you understand. I wanted to come out personally and explain.
"I did everything I could. I took pain-killers, had rest until the very end, but I just cannot compete at this level with Novak.
"At my age it would've been a risk.
"It's not that much of a surprise. This back spasm, whatever it might be, it's just not a fun thing to have during the day. It's just uncomfortable. But I'm positive and I'm hopeful that it's going to go away soon."