Novak Djokovic has expressed his pride after successfully retaining his ATP World Tour Finals title on Monday.

The Serb defeated world number one Rafael Nadal in straight sets at The O2 to win his third season-ending title and extend his current unbeaten run to 22 matches.

"It's the highest quality of tennis that you have next to Grand Slams, because every match that you play, you play against a top 10 player every second day," Djokovic to reporters. "Especially this year, with back to back Paris Bercy tournament and London, wasn't easy for sure.

"I am very proud of the way that I managed to recover and perform better and better as the tournament went on.

"I had the tough three setters against all of the opponents in my group, and then I managed to raise the level of tennis in the semi-finals and finals when it mattered the most."

Djokovic will now represent Serbia in the Davis Cup in Belgrade on Friday.