Williams, seeded 13th, downed Israel's 14th-seeded Peer 7-6 (7/4), 6-1 in a rematch of their epic quarterfinal clash in Melbourne in January, when Williams triumphed en route to her surprise Aussie Open championship.

Henin, seeded second here, had already advanced with a 6-2, 6-3 victory over ninth-seeded Russian Anna Chakvetadze.

The Belgian, who took over the world number one ranking earlier this week, improved to 15-1 in 2007.

She'll be meeting Williams for the first time since the Wimbledon semi-finals in 2003, when the American posted a straight-set victory.

"It's a long time we didn't play each other," Henin said. "She looks like she's healthy now -- that's the biggest difference -- and very focused on her tennis."

Williams is 14-1 this season, displaying a renewed commitment to the sport after over a year of injuries and distractions.

Playing in her first event since winning the Australian Open, she clearly has not missed a beat, posting straight-sets victories in each of her first four matches.

"I wasn't really surprised the way she came back in Australia," Henin said. "When she's healthy and really focused, she's a great champion. She has nothing to prove to anyone. She did it in the past."