Russian teenager Maria Sharapova [Images] said she was exhausted by her fight-back to force a third set in her loss to Kim Clijsters [Images] in Friday's US Open semifinals and must improve her physical conditioning to reach her tennis peak.

"I kind of gave it all I had in the tiebreaker," the top-seeded Russian said after her 6-2, 6-7, 6-3 defeat. "Then the third, I kind of ran out of gas.

"At that point it's pretty tough because physically I still have to get a lot better and I have to play these three-setters. The points are going to go on and on and on.

"This is not something that's going to happen overnight. I'm only 18 and this is going to take time. My body is still growing and I'm still adjusting to my own body."

The 18-year-old Sharapova, who exploded on the tennis scene with her 2004 Wimbledon victory at age 17, will reassume the number one ranking after the Open. Still, she recognises the need to get fitter.

"When the points started getting longer and longer, I felt that she had the advantage," Sharapova said of fourth-seeded Belgian Clijsters, 22.

"She was able to execute them well. I did have opportunities, going into the net, and she came up with great shots," added the Russian, who fought off five match points in the last game of the second set to force a tiebreaker.

Asked to describe Clijsters, Sharapova said, "Great athleticism. She moves amazingly, amazingly. That's a big plus in tennis."

Sharapova said her best is yet to come.

"When the time comes, the time's right, then it will happen. It takes a lot of work and I'm willing to go back on the practice court and in the gym, whatever I need to do, and work hard to win the Grand Slams."

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