Serena Williams wins in Rome

ROME -- Serena Williams rallied past Nadia Petrova of Russia 4-6, 6-2, 6-3 to extend her winning streak to 15 matches in second-round action at the Italian Open on Wednesday.

Williams has won titles in Charleston, S.C., and Madrid -- plus two victories in Fed Cup.

Serena's sister, Venus Williams, had an easier time reaching the third round, eliminating Ekaterina Makarova of Russia 7-6 (7), 6-1.

Considering all the tennis Serena Williams has played during her streak, she didn't have much time after finishing her first-round win over Galina Voskoboeva near midnight Tuesday.

"It wasn't that easy. It was a quick turnaround," Serena said. "When I saw the schedule, I said, 'Oh my God, are you serious?' That's life. You just have to be able to adjust and make the best of it, and so I was really happy to come through today."

However, Serena appeared to struggle with her back at times.

"Everyone has aches and pains here and there, and today I felt it a lot because I have been playing a lot and practicing even more," she said.

Williams next faces fifth-seeded Sam Stosur, who defeated Sara Errani 6-3, 7-5.

There were several upsets in women's play, with No. 3 Agnieszka Radwanska, No. 6 Caroline Wozniacki and No. 7 Marion Bartoli each losing.

Radwanska was beaten by 28th-ranked Petra Cetkovska of the Czech Republic 6-4, 4-6, 6-1; Wozniacki retired with upper respiratory illness while trailing 6-4, 4-0 to Anabel Medina Garrigues of Spain; and Bartoli was eliminated 6-3, 6-1 by Julia Goerges of Germany.

Also, French Open winner Li Na of China beat Iveta Benesova of the Czech Republic 6-1, 7-5; and 12th-seeded Angelique Kerber of Germany eliminated Vania King of the United States 6-3, 6-2.

Former French Open winner Ana Ivanovic routed Spanish qualifier Silvia Soler-Espinosa 6-2, 6-1 to set up a matchup with No. 2 Maria Sharapova.

Information from The Associated Press was used in this report.