Spanish tennis star Rafael Nadal said in an interview published by Vanity Fair that he was not sure "how long" he would keep "playing tennis" and that he was enjoying "one of the best seasons" of his career "until the knee injury."

"I don't know how long I will keep playing tennis. I'll be 31 in five years and taking into account the fact that I started at 16 ... Perhaps stopping now will help extend my career a little bit more. Until I had the problems with the knee again, the final at Roland Garros, had been one of the best seasons of my life. I felt able to win any competition. Complicated times came later," Nadal said.

The 26-year-old tennis star discussed the matches he lost to Swiss great Roger Federer at Wimbledon in 2006 and 2007, and what it meant to beat him a year later and take the gold medal at the Beijing Olympics to become No. 1 in the world.

"In 2006, I was very happy to have made it to the final, I didn't even go into it with the feeling that I could win it. Losing in 2007 was really hard. I had played very well during the whole match and in the fourth set, when I was winning, I felt a stab in the knee and lost my concentration. And I played the fifth set very badly. When you lose, you don't know if it's your last opportunity to win Wimbledon, it really affected me," Nadal said.

"Winning helps mentally in the following months, you feel that you have overcome an obstacle that you can jump over again. You get to your bedroom, see the trophy and you're proud of having gotten it. But it's not something permanent," the 11-time Grand Slam champion said.

"Success" is what "you fight to win," not the victory itself, Nadal said in response to a question about the biggest triumph of his career.

"Success is not the victory, it's what you've done to win. The knowledge that you've done everything in your power to achieve what you wanted. That feeling makes me very happy. This year I lost the final in Australia and I didn't like it, but I was happy in some ways. It was a success to have lost like that," the tennis star said.

Nadal began the summer on a high note by capturing a record seventh French Open title with a victory over then-world No. 1 Novak Djokovic of Serbia in the final.

He said in a Twitter posting in early September that he would be sidelined for the next two months with lingering knee problems.

The tennis star's knee injury, known as Hoffa's syndrome, or fat pad impingement, produces pain and swelling around the bottom and under the kneecap.

Nadal has not played since his shocking second-round loss at Wimbledon to Czech journeyman Lukas Rosol, considered one of the biggest upsets in the history of that prestigious tournament.

He missed the London Olympic Games, the ATP World Tour Masters 1000 events in Toronto and Cincinnati, the U.S. Open and Spain's Davis Cup semifinal tie with the United States over the weekend.

The Spanish star, known for his physically demanding style of play and incredible defensive skills, has struggled to stay healthy during his career, having been forced to withdraw from each of the four Grand Slam events due to various injuries. EFE