Roger Federer believes shot selection let him down in the ATP World Tour Finals decider against Novak Djokovic as he was beaten 6-3 6-4 in London.

The Serbian top seed claimed the season-ending title for a fourth consecutive year as Federer handed up 31 unforced errors to his opponent.

Djokovic, 28, ends the season with 11 title victories, including three grand slams - and it was decision making on court which caused the outcome at the O2 Arena, according to Federer.

"It would have been nice to serve a little bit better early on in the match, play better overall on his second serve, because he does allow you to play on his second serve," he said.

"Maybe at times I went for too much. The moments where I should have gone safe, I didn't, and vice versa. Those are the two regrets I have.

"If I played the match again, that is what I would try to do different. Other than that, I thought it was a good match. It was close.

"First six games were tough, to be down 4-2. I had my chances to at least be even. But I thought he played well. Still high-quality match, I thought."

Federer believes it will be tough for Djokovic to replicate his sensational form next year, and claims only minor changes could have a huge impact.

"It's hard to do. It's hard to play at this pace all the time. Doesn't need much, you know. We're talking margins," he said.

"You don't win a break point, he does, vice versa, it changes the whole outcome of the match. You can't always be on the winning side. Margins are small at the very top. That's why this year of Novak's is amazing.

"Rafa [Nadal] has been there. I've been there. We both know how hard it is to back it up year after year. It's not the first good year of Novak. Clearly he's going into next year with massive confidence.

"He's playing great. It's going to be hard for him to play a bad year, that's for sure. But this extremely good, it's always tough. But right now you would think it is somewhat possible, but you can't just repeat a year like this. It takes a lot of effort.

"You've got to be physically in shape, no injuries whatsoever. Mentally you have to be at your peak at all times. It's not as easy as it seems sometimes."