Novak Djokovic outclassed Roger Federer in the ATP World Tour Finals finale to cap what has been an incredible 2015 for the world number one.

Barring the one tournament in Doha – when he was sent packing by Ivo Karlovic in the quarterfinals – Djokovic has reached the final of every single tournament he has been in in 2015, and it need hardly be said, while winning most of those.

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Having clinched three of the four Grand Slams this year, Djokovic beat Rafael Nadal in the semifinal and then Federer, the man who beat him in the group stages on Tuesday, in the final to clinch a fourth consecutive year-end title in London.

"I guess Karlovic is my nemesis. I have a negative score against him," Djokovic said with a cheeky smile. "I have to talk about him before Doha next year. "It's been an incredible season.

"Other than that tournament, I've played all finals. Obviously, sitting here with this trophy alongside me, I couldn't ask for a better finish to the season. The last four years I managed to win the World Tour Finals, where the best players in the world are playing.

"For some reason or another, I've been playing some of my best tennis after the US Open, in Asia and also indoors, both Paris and London.

"I've been trying to really pay as equal attention to the work and the recovery, as well, mental and physical. It allows me to have the longevity. It allows me to have the matches and the tournaments I've had in the last couple years.

"But this season definitely stands out. I can't say I expected it, not at all. Obviously it gives me a lot of confidence for anything that is coming in the future."

What Djokovic will want more than anything else in the future is a French Open title, the one Grand Slam he has failed to win so far. Having already achieved so much in his career, which includes 10 majors, Djokovic will know if he cannot pick up that Roland Garros title, it will haunt him forever.

The Serbian has come close on quite a few occasions, and when he looked destined to finally go all the way, having dispatched of the near-unbeatable-on-clay Nadal himself, Stanislas Wawrinka played one of the matches of his life.

"Roland Garros is always one of the biggest challenges I have every year, but it's not the only one," Djokovic added. "There are the Olympic Games that are happening every four years.

"I will try to do as well as I've done in the last couple of years, always peak at the right moments and always try to perform my best at the biggest events.

"Now what I'm thinking about is the rest. I need some time to really recharge my batteries and then I'll think about my next season."