ATP World Finals: Tim Henman on Novak Djokovic's 'special' season

Novak Djokovic's victory Roger Federer at the O2 Arena on Monday night brings to an end another incredible season for the world number one.

He might have won three Grand Slams and had a 43-match winning streak in 2011, but in some ways coming back to secure the end-of-year number one spot again in 2012 is an even more impressive achievement.

Federer got off to the most unbelievable start in the final, he came out of the gate firing on all cylinders but Djokovic didn't panic, he dug in and for those two guys to play two sets in two-and-a-quarter hours shows the quality and how close it was.

It was a very fitting match to finish what has been an unbelievable year and it was a very fitting last shot, Djokovic under pressure out wide, able to find the passing shot and get the victory.

I think the game's stronger than it's ever been, so Djokovic maintaining his place at the top of the game after last year, when he just dominated everything and was the clear number one, is something really special. It's a fitting reward he's been able to win this Championship.

The margins between Djokovic, Federer, Andy Murray and Rafael Nadal are so small, it comes down to just a few points when they play each other. I think that's the beauty of their rivalry, that they're all capable of beating each other, which was emphasised by each of them winning one of the four Slams in 2012.

And I definitely still include Nadal in the conversation about the world's best, even though he hasn't played since Wimbledon. I really do keep my fingers and toes crossed that he will get over his knee injury and be back for the Australian Open at the start of next year, because it's not quite the same without him.

In the meantime, Djokovic has shown time and again that over the course of a year, he's had the ability to win the important points in the big matches. That's how the top four have separated themselves from the others, by being so consistent in the big events, and none more so than the world number one.

We've seen it again this week at the O2, where he's twice come back from a set and a break down, against Andy and Juan Martin del Potro.

As an opponent you know he's not going to lose it for you, you're going to have to put him away. Obviously he doesn't want to be in that position - he's not going out there trying to go a set and break down - but he just has enormous belief.

When you've done something time and time again, you get a lot of confidence out of that. Everyone now knows that Djokovic is not beaten until the last point is won.

I thought his performance in Sunday's semi-final was fantastic, because Del Potro played as well as I've seen him play for a set and a quarter.

There was no panic, he knew he was in a tight spot but he did a great job of breaking straight back and then really started to work Del Potro, moving him from side to side and hitting down the line - over the high part of the net with less court to aim into - so effortlessly.

I don't think you can just hit through him for a whole match with the way that he moves. If you can hit the slice to keep the ball low, serve well and keep the percentages high, so you don't give him too many looks at second serves, you have a chance.

But I'd say, right now, beating Djokovic is the toughest challenge in tennis.