Serena triumphed in damp and blustery conditions late on Friday

Eighth seed Williams is the only surviving American in the singles at Roland Garros following sister Venus's defeat earlier on Friday.

She will now face Russian 10th seed Dinara Safina for a place in the quarter-finals.

Should she progress, Williams could face top seed and defending champion Justine Henin in the final eight.

"I didn't think I played well at all actually. I thought I could have done a lot better," said Williams. "But I'm trying not to peak too soon. At some point I'm going to start playing better."

Belgian Henin cruised into the fourth round with a 6-2 6-3 win over Mara Santangelo.

It was the perfect way for her to celebrate her 25th birthday and set up a fourth-round tie against Austria's Sybille Bammer.

I wasn't feeling that well, I was cramping a little bit and had stomach problems. I just tried to fight for every point Justine Henin

Both players looked clumsy at first and had traded early breaks when Henin managed the telling one in the sixth game, before wrapping up the set with a backhand pass to seal her third break of the set.

And the Belgian, bidding for a fourth title here and looking to become the first woman since Monica Seles in 1992, and only the second in the professional era, to win the title in three successive years, broke the 25-year-old Santangelo straight away in the second set.

The Italian broke back but her resistance soon faded as she dropped three games in a row as Henin coasted through.

But Henin insisted the win was not as easy as it appeared, saying: "I wasn't feeling that well, I was cramping a little bit and had stomach problems. I just tried to fight for every point.

"I'm happy the match is over, that we could finish it tonight."

Austria's Sybille Bammer will meet Henin in the fourth round after she beat 16th seed Li Na 6-4 6-3.

Last year's semi-finalist Nicole Vaidisova reached the last 16 with a routine 6-4 6-4 win over Australian Samantha Stosur.

Vaidisova will now meet Italy's Tathiana Garbin, who trounced Stephanie Cohen-Aloro of France 6-3 6-0.

But former finalist Elena Dementieva is out after a 6-2 6-4 defeat to Marion Bartoli of France - which featured 14 double faults from the Russian.