The French Open has been pretty much a closed competition ever since Rafael Nadal won his first title on the red clay surfaces at Roland Garros in 2005.

Nadal has won nine of the last 10 French Opens, compiling a majestical record of 66-1. His only loss came in the semifinals of 2009 to Robin Soderling, who was then beaten for the title by Roger Federer.

But as the French Open begins on Sunday, the King of Clay is finally looking vulnerable. Nadal has lost five times on his favorite surface this season, and has fallen out of the top 5 in the world rankings for the first time in 10 years. Now ranked No. 7 and seeded sixth, Nadal even addressed the possibility of losing at the French Open after his last loss on clay to Stan Wawrinka in Rome.

"I am ready to accept the challenge," Nadal said. "If I go to Roland Garros [and] I lose [and] I don't play well, life continues. It's not the end of the world. I won so many times there. I don't want to [win] 15 Roland Garros. That's for sure. It's normal that I can lose. Losing is part of life."

But, he added: "I am sure that I can be competitive."

Martina Navratilova is sure of that, too. The Hall of Famer and Tennis Channel commentator doesn't want anyone to write off Nadal just yet, even though she does have her doubts about Nadal's physical well being and confidence. Nadal has fought through many injuries in recent seasons, and had an appendectomy last fall.

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"He seems to me to be a little bit less physically imposing," Navratilova said last week on a conference call. "I don't know if it's just my imagination. [He] just doesn't seem to be as muscular as he was five or six years ago . . . Maybe he trains differently because of his injuries, he can't train as hard as he used to."

And, Navratilova said, other players are stepping up. World No. 1 Novak Djokovic has a victory on clay over Nadal this season, and Andy Murray beat Nadal for the first time on the surface. Djokovic is looking as invincible as Nadal, or Federer, once was. In the draw made on Friday, Nadal could meet Djokovic in the quarterfinals.

"I think it's the other players are playing better and hitting a lot more topspin on the ball, hitting the ball harder, which does not give him the time to run around his backhand and dictate with the forehand . . . He can't park himself on the right side of the court," said Navratilova. "And also, by his own admission, he gets more nervous now. When he does get more nervous, his forehand goes shorter. Even when he does get to hit the forehand, he doesn't hit it with as much depth, and maybe power."

Given all that, Navratilova isn't down on Nadal's chances. "It could be he's just having a bad year or six months," Navratilova said. "Roger Federer said himself, until Rafa loses at the French he still has to be [the] favorite. You can't just throw out the last 10 years based on the last few months."

Still, Navratilova believes that the other players are no longer intimidated by the 14-time Grand Slam champion.

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"Certainly he's, I'm sure, feeling most vulnerable and he's looking most vulnerable," Navratilova said. "And that gives the other guys confidence when they play him. Before, it was like I don't want to get embarrassed playing Rafa and now they think they have a chance."

Soderling, whose career crawled to a halt in 2011 with injuries and illness, won't mind if someone beats Nadal at the French this year. That 2009 win over Nadal seems to define his career, though he's proud of many other things he's accomplished.

"So maybe it's better if Rafa loses again," he said this spring. "Then everybody will stop asking me about it."