Rafael Nadal confesses his greatest achievement is returning to the top of the world rankings, after suffering and combating a career-threatening knee injury.

The Spaniard is currently in London for the ATP World Tour Finals, and is on the verge of topping a comeback year by finishing the season being number one in the world.

It would be an incredible feat for the 13-time Grand Slam champion, especially seeing as he was sidelined with knee tendinitis for seven months, making his comeback early this year.

The four-time Davis Cup winner has struggled with persistent knee trouble for much of his illustrious career, but found form to claim the French and US Open, along with several masters events.

The current world number one told sport360: "It is the most difficult thing I did in my career. I'm 100 percent sure of that.

"After seven months of not even being able to practice tennis I came back and won from the beginning and was very soon able to have success in the best tournaments and against the best players.

"Sure I had doubts if I would come back like this. The doubts I had were normal. They are part of life."

Nadal is currently vying with Novak Djokovic for the top spot in tennis, and after claiming victory against David Ferrer earlier today, he just needs a win over Tomas Berdych or Stanislas Wawrinka to retain top spot.

Nadal has never won the Tour Finals, being beaten in his only final appearance by rival Roger Federer in 2010.

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