KEY BISCAYNE, Fla. — On a sultry afternoon in the subtropics, Serena Williams sat under an umbrella and sipped a refreshment.

A day at the beach? Almost. When play resumed, Williams made the world's top-ranked woman look like a beginner, beating nemesis Justine Henin 6-2, 6-0 in the quarterfinals of the Sony Ericsson Open.

Williams won the final 10 games today by playing nearly flawless tennis, while Henin unraveled after she double-faulted three times to fall behind 4-2.

"It wasn't easy," Williams said. "It was just the fact of me making the right shots at the right time."

Top-ranked Roger Federer advanced to the men's quarterfinals by beating Jose Acasuso 7-6 (6), 6-2. Federer, still seeking his first tournament title this year, had 31 winners and only 11 unforced errors.

Williams' rout of Henin a stunner, given the drama usually generated by the rivalry. Williams had lost their past three meetings, all in 2007, at the French Open, Wimbledon and the U.S. Open.

But Williams beat Henin in the Key Biscayne final a year ago, overcoming two championship points.

"I wish this was a Grand Slam," Williams said with a laugh. "I'm going to try do this at a Grand Slam, and not only at this tournament."

Williams could meet sister Venus in the semifinals Thursday. The older Williams, a three-time Key Biscayne champion, was to play No. 3-seeded Svetlana Kuznetsova tonight.

Conditioning can be an issue for Serena Williams, but she has never looked more agile than in the past week. No longer forced to rely on mere power, she lobbed over Henin to stay in rallies, chased down a drop shot to hit a winner and even playing serve and volley.

During her post-match news conference, an exchange regarding Williams' fitness left the room in laughter.

Reporter: "So many people here at the tournament have been buzzing about just how good you look physically on the court. You've not looked this good since... Can you finish that sentence?"

Williams: "Since 1982."

Reporter: "Serious question."

Williams: "That was a serious answer."

Seeking her fifth Key Biscayne title, Williams is improving as the tournament progresses. She committed 60 unforced errors in her opening match but had only 15 against Henin.

The Belgian, meanwhile, struggled even with her signature backhand and found herself stuck at the baseline.

"I was too defensive, for sure," Henin said. "Against this kind of player, you don't have any chance if you play that game, and I didn't have really any courage to do something else. When you're on the court, it's not that easy. Now I can say it wasn't enough."

On the men's side, James Blake wobbled at the finish but beat Radek Stepanek 6-3, 6-4. The No. 9-seeded Blake needed six match points in the final game to close out the victory.

By reaching the final eight, Blake matched his best showing at Key Biscayne. He has often struggled in the event, where his career record is only 11-6, including a loss in his opening match last year.

No. 15 Guillermo Canas, last year's runner-up, lost to No. 31 Igor Andreev 6-4, 7-6 (6).