China's Li Na looms as the greatest threat to defending French Open champion Serena Williams at Roland Garros. Li won the year's first Grand Slam title at the Australian Open in January. [Photo/Agencies]

The American powerhouse claimed a second Roland Garros crown last year-11 years after her first-and warmed up for the defense of her crown by winning the Rome title last Sunday, dropping just one set and 22 games all week.

It was a return to the form that helped Williams win 11 titles in 2013 and proof, if any was needed, that the fire within burns as ferociously as ever as she returns to the city she calls her second home, chasing an 18th major singles that would tie her with Chris Evert and Martina Navratilova.

Former French Open champion Li Na is probably her main rival as the Chinese attempts to become the first player since Jennifer Capriati in 2001 to win the year's first two Slams.

Maria Sharapova, winner in 2012 and runner-up last year, will also be confident of a strong run after beating Li on the way to claiming the Madrid title this month.

Poland's gritty world No 3 Agnieszka Radwanska, and Serbian duo Jelena Jankovic and Ana Ivanovic, the champion in 2008, will also be strong contenders while Simona Halep, the aggressive Romanian baseliner who has blasted into the top four, is more than capable of causing an upset.

Whoever Williams faces, however, she will not be feeling any extra pressure just because she is defending champion.

"Usually I'm like, 'Oh my God, I have to defend'. This time I'm going to be cool with it," Williams, who has bought a second Paris apartment near the Eiffel Tower, said recently when looking ahead to the French Open.

"I've been feeling that way for a while now. I think that's a good way to feel, since I won so many titles last year."

Ominously for her rivals, Williams' previous French Open triumphs both came following a title run in Rome.

Former semifinalist and reigning Wimbledon champion Marion Bartoli, now retired, said Williams's match temperament was her biggest asset and the reason she still sets the standard.

"I'm so impressed with how focused she is going into every Grand Slam," Bartoli, who will work as a commentator for ITV and Eurosport this year, said.

"She doesn't act like she's a favorite. She is just focused on winning every single match, which is why she is a champion. She goes into every match thinking she needs to play her best tennis, not that she's the favorite."

Gone are the days, it seems, when a teenager would jump out of the pack to capture a Grand Slam.

Claycourt battles

World No 2 Li is also 32 and, like Williams, appears to still be improving. She showed with her magnificent 2011 run in Paris when becoming the first player from an Asian country to win a Grand Slam singles title that she has the tools and the temperament to survive two weeks of claycourt battles.

A second-round defeat last year to American Bethanie Mattek-Sands left her at a low ebb but she rebounded impressively, reaching at least the quarterfinals in 13 of her next 14 tournaments and winning this year's Australian Open.

"I'm feeling pretty good," Li said during her run to the quarterfinals in Rome last week." I'm playing some high-quality matches. And I'm feeling pretty loose-I'm just going to do my best out there."

Sharapova, now comfortable sliding around on clay after looking clumsy on it earlier in her career, will benefit from being slightly under the radar this year after a shoulder injury meant a slow start to the year.

The former world No 1, now down at eighth in the WTA rankings, returned to her best form once the claycourt season started, winning back-to-back titles in Stuttgart and Madrid.

"I've done a really good job of transitioning from the hard to the clay and really improving physically and recovering well from match to match," the Russian told the WTA's website.

"I've benefited from that in the last couple of years. I enjoy playing on all surfaces. But I'm really happy about changing my results on clay the last few years."

A dark horse for the title could be Romanian Halep, who is the most improved player on the Tour over the past couple of years.

Wins against Ivanovic and Petra Kvitova on the way to the final in Madrid have given her plenty of confidence as she bids to avoid a hat-trick of first-round exits at Roland Garros.