A surprisingly resurgent Serena Williams has wound back the clock to seize French Open favouritism a decade after raising the trophy in Paris for the only time.

Even her arch-rival, Australian title hope Samantha Stosur, is at a loss to explain Williams' dramatic transformation on her least-preferred surface, but admits the 13-times major winner has been virtually unstoppable during her unbeaten clay court run this year.

Williams has conceded only two sets in 17 matches since collecting her first title on clay in four years - and only the fourth of her career - in North Carolina last month.

The 30-year-old also reigned in Madrid to climb back into the world's top five after spending 11 months out of the game in 2010-11.


"I don't know what it is - she's the only one who can answer that - but she seems super dedicated at the moment, maybe more so than in recent years," said Stosur.

"She's obviously very focused on her own goals that she's trying to achieve, and in the matches I've played against her, and the ones I've seen her play, she's playing very, very well."

Williams lost to Stosur on a hard court in last year's US Open final, but warned her rivals she is a different beast almost a year into her comeback from injury and illness.

"I'm feeling better on clay than I did at the US Open," the American said. "I have a better ranking and my fitness is better.

"I just feel better this time around. I feel I can play on any surface and that's the right attitude for me. I'm enjoying my tennis. This is where I belong and what I do best."

A younger veteran, 25-year-old Maria Sharapova, is challenging Williams for favouritism after winning in Stuttgart and defending her title last week in Rome.

Like Williams, Sharapova has had quite a renaissance for a player who once famously compared her movement on the slippy red dirt surface to "like a cow on ice".

"I have improved," said the world No 2, who is chasing the only Grand Slam title still to elude her.

"Most of it comes down to the physical aspect and patience and not changing my game but relying on the things I have improved, like sliding and playing and definitely on the serve, and this is helping."

After setting an impossible pace to start the season, world No 1 Victoria Azarenka has had a form dip in recent times but the top seed will still take a power of beating in Paris.

Azarenka romped to four titles, including her first Grand Slam success at the Australian Open, during a 26-match winning run.

But she withdrew from her third-round match in Rome with a shoulder injury and has also has been troubled by a sore wrist.