Williams, who is playing only her second tournament on clay, joins older sister Venus, who beat her in the first - last month's Italian Open.

Another 104mph bullet hits the court

"A lot of people say you have to rally on the clay," she said, "but I think it's really exagerrated what they say. I don't feel my game has to change at all on the clay."

But Serena will have a tough next match, against No. 4 seed Arantxa Sanchez Vicario, a 6-2, 6-1 winner over Alexandra Fusai.

The 16-year-old Williams drew gasps from fans as she slammed serves and backhand returns past Van Roost, a Belgian ranked 14th in the world, 13 spots higher than Williams.

Williams allowed Van Roost to win just four points in her first five service games.

The American teenager moved about five feet inside the baseline on Van Roost's second serves, intimidating the Belgian into six double faults.

Sanchez Vicario: next opponent for Willams junior

Sanchez Vicario, a two-time French Open champion, said she must be patient against the powerful Williams.

"I will have to keep her on the run, so she does not have a chance to hit the ball hard," the Spaniard said. "Here, the points are a little longer."

Also advancing to the fourth round was the No. 7 seed Conchita Martinez, a 6-0, 6-1 winner over another Spaniard Virginia Ruano-Pascual.

Iva Majoli, the No.10 seed and defending champion, won 6-2, 7-6 (7-2) over Mariana Diaz-Oliva of Argentina and she was joined by local favourite and No. 14 seed, Sandrine Testud of France.

Another seed falls

In the men's tournament, another French player was successful, with yet another top seed going out.

The local victor was Cedric Pioline, who defeated the No. 10 seed, Richard Krajicek of the Netherlands, 6-3, 6-2, 7-5.

Top man: Rios is clear men's favourite now

Just one of the original top 10, No. 3 Marcelo Rios of Chile, remains in the tournament.

The No.16 seed, Alberto Berasategui is still there, after beating Slovakia's Dominik Hrbaty 6-7 (2-7) 6-3 7-5 7-5.

And the man who beat the defending champion Gustavo Kuerten is also through to the fourth round.