Petra Kvitova finds her form at French Open as Serena Williams digs deep to advance

​Wimbledon champion Petra Kvitova powered into the last 16 at the French Open on Saturday, finally finding her form after earlier struggles at Roland Garros.

The fourth seed from the Czech Republic outplayed Romanian Irina-Camelia Begu 6-3 6-2 in just 58 minutes, taking the last five games at a canter.

That was in stark contrast to her first two rounds that saw her grind out six sets and spend just four minutes short of five hours on the court.

"I think it was for sure the best match which I played here so far, so I'm really glad for it," Kvitova said.

"I'm glad that I didn't forget many things from my game and it's still there. I felt better today, for sure. It's the third round which I didn't pass here last year, so I was really motivated by it."

Next up for Kvitova, who also won Wimbledon in 2011, but who has only managed a single semi-final in Paris in 2012, will be a fourth round tie against Timea Bacsinszky of Switzerland who upset 16th seeded American Madison Keys 6-4 6-2.

Sloane Stephens revived flagging American hopes by seeing off Bulgaria's Tsevtana Pironkova 6-4 6-1, winning eight of the last nine games to clinch the victory.

Stephens, who has clawed her way back up the world rankings after a bad wrist injury sidelined her late last year, will now look forward to a fourth round clash with top seed and compatriot Serena Williams.

Late in the day they were the only two remaining US women remaining in the draw out of 17 starters.

Williams digs deep

Williams battled back to advance into the last 16 with a 3-6 6-4 6-2 win against Belarussian 27th seed Victoria Azarenka Court Philippe Chatrier.

The 33-year-old American was staring defeat in the face at a set and 2-4 down in the second before finding her range in the nick of time.

She was also a break down early in the deciding set, but by that time she had found her range and she duly won six games in a row to clinch the win as she continues in search of a 20th Grand Slam title.

"Today it was not easy, Victoria played very well and I was struggling," said the two-time champion.

"She won the first set and I just said to myself 'allez, allez Serena, come-on."

The two have played each other 19 times now with 19-time Grand Slam winner Williams well ahead at 16-3.

Errani revenge

Also through to the last 16 in early play was Italian Sara Errani, losing finalist in 2012, who gained revenge on Andrea Petkovic of Germany 6-3 6-3.

Petkovic ousted Errani at Roland Garros last year at the quarter-final stage before losing in the semis.

There was disappointment though for another Italian, 2010 Roland Garros champion Francesca Schiavone losing 7-5 6-4 to Andreea Mitu of Romania.

Ranked 100th in the world, Mitu is playing in her first French Open and her three wins in Paris this week were her first at Grand Slam level. 

Her next opponent, with a quarter-final slot at stake will be 93rd-ranked Alison Van Uytvanck of Belgium, who ended the hopes of France's Kristina Mladenovic 6-4 6-1.

Van Uytvanck was 1-5 in Grand Slam matches coming into Roland Garros, but like Mitu she has been able to benefit in her quarter of the draw from the first round defeat of sixth seed Eugenie Bouchard at the hands of Mladenovic.

"It was not easy because she is a very good player and I was very nervous in the beginning," said Van Uytvanck.

"It's a big dream come true. I was happy enough winning in the first round and now being in the fourth round I can't believe it.

"Maybe I am the favourite in the fourth round. I think she (Mitu) is very confident. She's a solid player. She is a big hitter. We have nothing to lose."