PARIS (AP) – Serena Williams, of all people, got a case of the jitters.

That was her explanation, anyway.

The 10-time Grand Slam champion kept finding herself in, then out of, trouble in the French Open quarterfinals Wednesday, until running out of stamina and strokes down the stretch of a 7-6 (3), 5-7, 7-5 loss to Svetlana Kuznetsova that ended Williams’ 18-match winning streak at major tournaments.

“I had an opportunity, and I got really tight, and I pretty much gave it to her,” said the second-seeded Williams, who blew a 3-1 lead in the third set. “It was like, ‘Here. Do you want to go to the semis? Because I don’t.’ She was like, ‘OK.”‘

The seventh-seeded Kuznetsova’s semifinal opponent Thursday is No. 30 Samantha Stosur of Australia, who defeated Sorana Cirstea of Romania 6-1, 6-3. The other women’s semifinal is No. 1 Dinara Safina of Russia against No. 20 Dominika Cibulkova of Slovakia.

If Stosur-Cirstea was as onesided as could be, Kuznetsova-Williams was hyper-competitive and superbly played. Until, at least, Kuznetsova took eight of the last nine points, breaking Williams in the final game.

“Honestly, I think I lost because of me,” Williams said, “and not because of anything she did.”

Williams denied she felt fatigued, blaming nerves instead.

How could that be?

“Maybe I put some expectations on myself that I didn’t put on myself initially,” she said.

Roger Federer spoke about dealing with nerves Wednesday, too, although his problems came before he began playing 11th-seeded Gael Monfils of France.

“We’re all nervous at this stage of the competition. I felt it. Yesterday I felt it, and I felt it again today in the warmup,” said Federer, who knows this might be his best chance to win the only Grand Slam tournament missing from his resume. “I was tired, I was nervous, and I didn’t feel really good. Then once out on court, you know, I get my act together.”

There’s an understatement.

Federer beat Monfils 7-6 (6), 6-2, 6-4 to close in on completing a career Grand Slam and earning a 14th major title to tie Pete Sampras’ career record.

Next up for Federer is No. 5 Juan Martin del Potro of Argentina, who eliminated No. 16 Tommy Robredo in straight sets. Friday’s other men’s semifinal will be No. 23 Robin Soderling – the man who upset four-time defending champion Rafael Nadal in the fourth round – against No. 12 Fernando Gonzalez.

Federer is 26-1 against the other semifinalists, including 5-0 against del Potro.

“Doesn’t mean because I have a great record against all the players left in the draw that, you know, I’m going to win this,” Federer said, “but I’ll definitely try everything I possibly can to do it.”

He is into his 20th consecutive major semifinal, extending his own record; del Potro is in the first of his career.

“We all know how he plays,” the 20-year-old del Potro said, “and we all know what he wants to achieve here now that Rafa is no longer here.”

The women’s semifinalists might very well be relieved to know that 2002 French Open champion Williams is gone, because she was by far the most accomplished of the remaining players.

Kuznetsova, the 2004 U.S. Open champion, is the only member of the remaining quartet to have won a major title. Neither Stosur nor Cibulkova has won any singles title on tour.

Still, Kuznetsova has something of a reputation as someone who is capable of folding at key moments of tense matches, including a loss to Williams at the Australian Open in January after serving for the match in the second set.

“Same scenario,” Kuznetsova said.

The Russian led 3-0 in each of the first two sets Wednesday, before allowing Williams to come back. Then Kuznetsova was a point from taking a 5-2 lead in the second set when she twisted her right ankle and tumbled to the court. She wound up caked with clay, from her head to her socks.

Kuznetsova recovered from that, though, and served for the match at 5-3 in the second set. Williams broke there, and again at 5-5, then served out the second set with a 114 mph ace.

Williams broke yet again to open the third set and appeared in control at 3-1. But leading 3-2, 40-love, she got broken back, contributing three unforced errors, including an ill-advised and poorly executed drop shot that landed wide to make it 3-all.

Williams leaned over and rested her forehead on the end of her racket. She came to the French Open without a lot of recent work, because of a bothersome left knee and a career-worst four-match losing streak, and that began to show.

“Not a lot of preparation,” said her mother and coach, Oracene Price. “She did the best she could do.”