LONDON (AP) -- Exactly one year ago, as the world's best tennis players faced each other in London at the season-ending finale, Rafael Nadal couldn't even practice.
Trying not to look too far ahead, the Spaniard was at home in Spain, just thinking about his badly injured knee. Nadal didn't know if he would play competitive tennis again.
''The thoughts about the future? Just focus on recover, focus on try to find a solution for my knee, and that's it. Working every day, trying to do different things to try to be back,'' the 13-time Grand Slam winner said Wednesday of his feelings at the time.
Things have changed dramatically.
Twelve months later in London, Nadal capped his remarkable comeback on Wednesday by clinching the year-end No. 1 spot for the third time in his career.
Just nine months after returning from his career-threatening injury, Nadal became the first player to hold, lose and regain the year-end No. 1 spot twice by beating Stanislas Wawrinka 7-6 (5), 7-6 (6) at the ATP finals. Roger Federer and Ivan Lendl did it once in their careers.
Nadal, who beat Wawrinka for the 12th time in as many matches, can't be overtaken by No. 2 Novak Djokovic.
It's the first time since 2010 that the Spaniard, who returned from a seven-month injury layoff in February, ends the year as the top-ranked player.
''It is one of the best things that I did in my career: come back to No. 1 after three seasons. That's very difficult in our sport, and after a very important injury,'' Nadal said after celebrating with fist pumps and leaps as if he had won the tournament - the only major event missing from his victory collection.
Nadal advanced to the semifinals of the year-end tournament because of his straight-sets victory over the Swiss.
The Spaniard's season will be remembered as the story of a stunning comeback during which he captured 10 titles - including two Grand Slams.
Following his defeat in the second round at Wimbledon last year and the extended absence that followed, only the boldest of bookmakers would have bet on Nadal achieving such success in 2013.
When he resumed playing, his left knee was still causing him pain. However, Nadal's confidence grew during a Latin-America tour which produced a 12-1 match record, including titles at Sao Paulo and Acapulco.
After securing his eighth French Open title, the only real disappointment of the year came at Wimbledon, where he lost to Belgian Steve Darcis in the first round - before adding a second US Open title.
The 27-year-old Spaniard also won five ATP World Tour Masters 1000 events and went undefeated on hard courts (26-0) through to the China Open final, where he lost to Djokovic.
Nadal, who overtook the Serb for the No. 1 ranking last month, said he started to think seriously about the No. 1 spot after his triumph at the U.S. Open.
''Because after winning five Masters 1000, two Grand Slams, all the things, I felt that I had everything to be there,'' he said. ''But at the same time, I have a zero in Australia, I have a zero in Miami, I have a zero in Wimbledon. A lot of zeros to be No. 1. I think it is a great effort because I have unbelievable competitors in front. That makes the year end No. 1 very, very special.''
Nadal beat David Ferrer in his opening match on Tuesday and kept the momentum going against Wawrinka.
After breaking him in the fifth game, he looked in complete control. Raising his game on important points, he erased Wawrinka's two first break points to build a 5-3 lead and looked set for an easy victory.
But the tournament debutant held on as Nadal served for the set, sending a sizzling crosscourt backhand that the top-ranked player returned out of court to earn a third break point. He seized his opportunity, wrongfooting Nadal with a crosscourt forehand.
The tie break was a tense affair. At 5-5, Nadal defended superbly to return a devastating forehand down the line from his opponent, who shanked his next shot. Nadal sealed the set at the first opportunity - as Wawrinka stumbled as he went for an easy volley before smashing his racket on the floor.
Nadal then converted the second of three break points in the fourth game of the second set as Wawrinka sailed a forehand long. But as in the first set, Wawrinka battled his way back into the match, winning his next service game at love before taking Nadal's serve for the second time with a volley that clipped the net cord and bounced over.
Both played more aggressively in the closing stages and held serve until a second tiebreak. Nadal made the most of Wawrinka's mistakes to take a 3-0 lead, but his gritty Swiss opponent took a series of risks with his returns and rallied to reach set point at 6-5. Nadal saw off the danger with a smash, pumping his fist in relief, before sealing victory on his first match point.
''He is playing better tennis than me,'' said Wawrinka, who has yet to win a set against Nadal. ''When it gets important in the match, for him it's just normal. He's just playing his best tennis or he is playing even better.''