PARIS — French Open runner-up Sam Stosur was knocked out of the tournament Friday, losing to Gisela Dulko 6-4, 1-6, 6-3 less than an hour after defending champion Francesca Schiavone became the first player to reach the fourth round.

Stosur struggled early then easily won the second set to take control against Dulko. But the 51st-ranked Argentine put herself back in the match with a break to take a 4-2 lead in the third set. She then held twice to advance.

Later Friday, Novak Djokovic is scheduled to put his perfect season on the line against 2009 U.S. Open champion Juan Martin del Potro. Roger Federer will also be back on court to play for a spot in the fourth round, as will top-seeded Caroline Wozniacki.

The eighth-seeded Stosur, who also reached the semifinals at Roland Garros in 2009, had 35 unforced errors in the match on Court Philippe Chatrier.

Dulko matched her best Grand Slam result by reaching the fourth round. She also won three matches at the 2006 French Open and the 2009 U.S. Open.

Schiavone was broken at the beginning of her match, then won five straight games to take the first set and eventually the match when opponent Peng Shuai of China retired while trailing 6-3, 1-2.

Peng called for a doctor after the first game of the second set and had her blood pressure checked. She looked exhausted in her chair, even starting to cry, but broke for a 2-0 lead.

Schiavone won the next game before her opponent quit.

Seventh-seeded David Ferrer also won, beating No. 31 Sergiy Stakhovsky of Ukraine 6-1, 6-1, 6-3.

On Thursday, Rafael Nadal didn’t exactly put his doubters to rest despite reaching the third round with a straight-sets victory.

The five-time champion, who needed all five sets in the first round, looked shaky again at Roland Garros but still managed to advance by beating Pablo Andujar 7-5, 6-3, 7-6 (4).

“Here I am not playing that well, and I’m going to try to play much better next time,” said Nadal, who trailed 5-1 in the third set before forcing the tiebreaker. “That’s what I can say, and that’s what I can think … I will see what’s going on after.”

Kim Clijsters and Maria Sharapova also struggled for long stretches Thursday, but it was only the second-seeded Clijsters that was eliminated from the tournament.

Nadal entered the tournament as the co-favorite along with Djokovic — the man who beat him in two clay-court finals this month. The Spaniard needs one more French Open title to equal Bjorn Borg’s record of six.

But so far Nadal has failed to dominate the way he did in the past, when he built a 38-1 record at Roland Garros over six years.

“The problems I think everybody knows; the solutions is more difficult,” Nadal said, explaining that he has not been aggressive enough. “Probably I am playing a little bit nervous, and that’s why (my) legs didn’t work as well as usual.”

Even Andujar, a 25-year-old Spaniard who has never reached the third round in eight Grand Slam appearances, noticed a weaker Nadal across the net.

“I think Nadal is going through perhaps a period of doubt,” said Andujar, who had eight set points in the third. “But little by little, he can gain confidence and build confidence.”

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