Paris, France - Down a set and a break to a former No. 1 player did not faze Serena Williams, who rallied past Victoria Azarenka and advanced to the fourth round of the French Open on Saturday.

The top-seeded Williams became the first woman in the Open Era to achieve 50- plus wins at all four Grand Slams, but it wasn't particularly easy.

She went down a set for the second straight match and lost a service game early in the second. The 19-time Grand Slam champion stayed poised and pounced when Azarenka showed signs of folding and pulled out a 3-6, 6-4, 6-2 victory in just over two hours.

Azarenka came in as a big underdog despite being one of Williams' closest rivals as recent as two years ago. She has made it past the quarterfinals in one tournament this year and has not won since Cincinnati in Aug. 2013.

A drop shot on break point gave Azarenka the advantage in the first set, and Williams won the final four games of the second to pull even.

Azarenka won the first two games of the deciding set, but Williams quickly recovered and controlled the rest of the match. Azarenka let out a deafening shriek when her serve was broken to go down 4-2, and on match point, Williams whipped a passing shot by the Belarusian.

Another American is awaiting Williams in the next round, as Sloane Stephens defeated Tsvetana Pironkova, 6-4, 6-1. Stephens has yet to drop a set in her three matches, which included knocking off Venus Williams in the first round.

She's bested the younger Williams once before at the 2013 Australian Open but has since lost three meetings, including at Indian Wells and Madrid this year.

Two-time Wimbledon champion Petra Kvitova cruised to a 6-3, 6-2 win over 30th- seeded Romanian Irina-Camelia Begu on Saturday. Kvitova, a semifinalist at Roland Garros in 2012, committed only nine unforced errors and did not face a break point.

"I think it was, for sure, the best match I've played here so far. I'm really glad for it," said Kvitova, who needed three sets to win each of her first two matches.

Up next for the fourth-seeded Czech will be Timea Bacsinszky of Switzerland. Bacsinszky, seeded 23rd, beat American Madison Keys 6-4, 6-2 to reach the round of 16 at a Grand Slam for the first time in her career. The 16th-seeded Keys had advanced to the semifinals at the Australian Open in January.

American Irina Falconi also lost her third-round match. She was beaten by German Julia Goerges 6-4, 6-1.

In other third-round play, former French Open finalist Sara Errani gained a measure of revenge against 10th-seeded German Andrea Petkovic with a 6-3, 6-3 win. The 17th-seeded Italian was ousted by Petkovic in the quarterfinals at Roland Garros last year.

Errani's countrywoman, Francesca Schiavone, failed to join her in the fourth round. The 2010 French Open champion lost 7-5, 6-4 to unseeded Romanian Andreea Mitu. Ranked 100th in the world, Mitu had never won a match at a major tournament before this week.

Belgian Alison Van Uytvanck advanced with a 6-4, 6-1 win over France's Kristina Mladenovic.