Rafael Nadal admits his return to the top of the world rankings after battling a serious knee injury is the greatest achievement of his illustrious career.
Nadal has arrived in London for the season-ending ATP World Tour Finals on the verge of capping a remarkable recovery by finishing the year at number one in the world.
That would be an incredible effort given the 13-time Grand Slam champion was sidelined with potentially career-threatening knee tendinitis for seven months from mid-2012 until early this year.
The 27-year-old Spaniard, who has struggled with knee problems for much of his career, returned to win the French Open and then the US Open, as well as Masters events in Indian Wells, Madrid, Rome, Montreal and Cincinnati before reclaiming number one spot last month.
"It is the most difficult thing I did in my career. I'm 100 percent sure of that," Nadal said.
"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.
"I have doubts before every match and much more when I am injured for seven months.
"I didn't know how the injury was going to improve or if I could get back on tour."
Nadal now needs to win two of his three Group A matches against David Ferrer, his opening opponent on Tuesday, Tomas Berdych and Stanislas Wawrinka to guarantee top spot ahead of world number two Novak Djokovic.
But he is adamant that regaining his position as the sport's preeminent force means less than the emotions he felt when he triumphed at the Grand Slams at Roland Garros and in New York.
"For me the emotions were very high in a lot of moments during a lot of tournaments, those moments were more special than being number one," he said.
"I already finished two years at number one in the rankings, so if that happens again great.
"But if not I don't think it is going to affect my standing in the history of tennis.
"I prefer being number one yes, but during the whole year I said a lot of times the number one is not a goal for me anymore.
"It was a goal for me in 2008 because then I felt I had a great career and I would be sad if I played that good and never got number one.
"Today the goal is to be healthy and competitive. It makes me happier to go to a good tournament and win it."
Nadal has never won the Tour Finals, losing his only final appearance to Roger Federer in 2010, and he has never hidden his discomfort at playing under lights on a fast court that doesn't suit his style.
After losing to Ferrer in the Paris Masters semi-finals on Saturday, Nadal is starting to feel the strain of such a gruelling year and he was quick to make defending champion Djokovic the favourite for the Tour Finals title.
"For the moment I didn't adapt very well to the hard courts. I played so-so last week in Paris, but it is the end of the season and a lot of the time that has been tough for me," Nadal added.
"I am a little bit more tired and these tournaments are sometimes not my best, but that doesn't mean I'm not trying my best in every match.
"I tried to play with the right attitude in Paris. I think my attitude was better than my tennis though!"