Rafael Nadal will end the year ranked number one in the world, 12 months after he feared he might never get back to his best.
The Spaniard, 27, missed seven months with a knee injury
before returning to action in February.
Since then, he has won 10 titles, including two Grand Slams - the
regained the top ranking from Novak Djokovic.
BBC Sport's Tim Henman examines Nadal's achievement and the game that got him there.
Nadal's rise to the top
Rafael Nadal is the equal of anyone in sport at being able to deal with adversity.
Nadal in 2013
- Returned from injury in February having dropped to number five in the world - his lowest ranking since 2005
- Went on to win two Grand Slams - the French Open and the US Open
- Also won eight other titles, including his 26th Masters title with victory in Cincinnati
- Currently has a career-best 73-6 year-to-date match record
- Ensured he finished as year-end world number one with victory over Stanislas Wawrinka in the World Tour Finals
- Became only the fifth player since rankings began in 1973 to move from outside the top three the previous season to finish number one the following season
- If he spends another two weeks at number one - to reach 109 - he will match Bjorn Borg in sixth place on the all-time list
He has the precious ability not to let the previous point affect the next one, and that is the most basic form of psychology in sport - stay in the present tense. I haven't seen anyone better.
Pete Sampras was good at not letting things distract him but in terms of being a complete player, capable of winning on all surfaces, Nadal is as good as any.
He's finished number one in the world before and to do it a third time is an incredible achievement in itself, but it makes it all the more significant given that he missed seven months with a knee injury.
I first heard about him when he was 16 years old and he beat Carlos Moya in a tournament, but the first time I really saw him was when we practised together in Miami.
He was probably 17 at the time and I'd never come across anyone that hit the ball that hard - and to be honest not always in the right direction.
He was using so much energy that I said to Paul Annacone, my coach at the time, 'How does he expect to play like this for two hours?' I didn't really think he could last for 20 minutes.
Then we played a practice set which he won 6-1 in about 15 minutes.
You knew that forehand was something special from the first time it came towards you. You'll never see anyone hit it as hard and with as much spin. That shot and the fact that he's a lefty are what make him so hard to play against.
I played him twice in competition, the first time in Rome in a night match on clay. I played really well and lost 6-2 6-2 - I was pretty happy with that. The other time was in Dubai and I didn't play so well. I served for the first set and probably should have got something out of that match.
Humble, focused and intense
It's been incredible to watch his rise in the years since then, but despite all the success he hasn't changed as a person.
How do you attack Nadal?
"You've got to be like Djokovic, who has a double-handed backhand and a body that is almost reptilian, with joints he can seemingly undo yet still find balance when he's pulled way out of court. You also need the fitness to stick around.
"He can also be more aggressive these days, something he first showed when he won the US Open in 2010, with more bite on the serve and flattening out the forehand.
"Nadal is playing as well as he's ever done, and the scary thing is that I still think he's not at 100% this week by any means."
He's incredibly humble but very focused and intense in what he's doing.
I'm not surprised that he managed to come back so successfully on clay this year, but to come back and win Indian Wells, to win back-to-back Masters on hard courts in the United States and
to win the US Open
was something special.
I don't think one should be surprised when you're talking about one of the greatest tennis players of all time, but that really did surprise me.
He'll have to manage his knee for the rest of his career but he does that very well.
And if he can continue to do so, it will be fascinating to see if he can get up to match Roger Federer's record 17 Grand Slam titles or perhaps even surpass it one day.
I tend to think 17 is still going to be tough to beat.
Barclays ATP World Tour Finals
- Venue: O2 Arena, London
- Date: 4-11 November
Coverage: BBC TV: One singles match each day live; BBC Radio: Every afternoon singles match live on 5 live sports extra; BBC Sport website: One singles match each day streamed online, live text commentary on every singles match