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.