KEY BISCAYNE, Fla. -- At the end, Serena Williams' shots were accompanied by grunts louder than her tangerine dress.

One last sequence of overpowering strokes gave Williams a victory she wanted badly, not so much because she advanced to the final of the Nasdaq-100 Open, but because she beat her older sister.

Serena routed Venus 6-2, 6-2 in a semifinal sibling showdown Thursday, calling the win the biggest of her career. Perhaps that's an overstatement, given that Serena won the U.S. Open in 1999.

Or perhaps not.

"I'm shaking a little," she said moments after the victory, which took her just 50 minutes to complete. "I can't believe I finally beat her. I'm like in shock."

For Serena, the win meant a breakthrough and a shift in the balance of power within the first family of women's tennis. She beat Venus for only the second time in their seven meetings, and her other victory came in the 1999 Grand Slam Cup, which she considered an exhibition.

"This is a big milestone for me and for all younger sisters and brothers out there," she said, laughing. "It definitely removed a mental block for me."

While there was no doubting Serena's motivation, Venus' performance was so listless that the crowd booed her briefly in the second set. Venus said she felt fine and merely had a bad day against a hot opponent.

"I just felt like I never really got into the match," said Venus, seeded second. "Before I knew it, it was over."

So is Venus' reign on Key Biscayne, where she had won 22 consecutive matches and titles in 1998, 1999 and 2001. Now it's eighth-seeded Serena with a shot at her first Key Biscayne championship.

Her opponent in the final Saturday will be the winner of Thursday's late semifinal between top-seeded Jennifer Capriati and No. 5 Monica Seles. Regardless of the results the rest of the week, Capriati is assured of retaining the No. 1 ranking until at least April 15.

In a matchup between the past two U.S. Open men's champions, top-seeded Lleyton Hewitt need 2 hours, 44 minutes to beat No. 6 Marat Safin 2-6, 6-2, 7-6 (4). Hewitt's opponent in the semifinals tonight will be No. 12 Roger Federer, who beat Andrei Pavel 6-1, 6-1.

The other semifinal today will pit No. 9 Andre Agassi against 1998 champion Marcelo Rios.

Williams-Williams VII generated none of the hype that accompanied their last meeting in the 2001 U.S. Open final, and the 14,000-seat stadium at Crandon Park was only one-third full when the match began.

Serena smacked winners on the first four points to break serve at love, and the rout was on. Venus rarely ventured to the net, where she can be most dangerous, and struggled to keep her groundstrokes in play. One shot sailed so wildly that Serena, standing at the baseline, had to leap out of the way.

And Serena feasted on Venus' tentative second serve. There were 16 such points, and Serena won all but one, often smacking her return for a winner.

"I don't care who's playing against me," Serena said. "Man, woman or dog, I'm going to attack their second serve."

As Serena's lead reached 3-1 in the second set, Venus looked increasingly lethargic.

"C'mon, Venus, wake up!" a fan shouted.

But she lost her serve for the fourth time to fall behind 4-1, and as she walked slowly to her chair, the crowd booed and whistled. There were more jeers two games later as Venus lost her serve yet again.

"I heard it," she said. "I got applause when I left, so that was OK."

As usual when Williams plays Williams, the match was hardly an artistic triumph, which probably contributed to the crowd's displeasure. With both sisters playing high-risk tennis, there were lots of errors and few long points. The nature of the matchup is such is that it has rarely been entertaining to watch.

"We're not out there hitting 100-ball rallies," Venus said. "That's not us."

Few points lasted longer than three or four shots, and many ended with a misfire by Venus.

"She was very erratic out there," Serena said. "She played the way I normally play."

When it ended, there was no repeat of the emotional scene following last year's U.S. Open final, which Venus won. On that occasion the sisters embraced and Serena cried. This time they met at the net with a perfunctory handshake like after any other match against any other opponent.

Their prematch routine was a family affair. Barely an hour before the start the sisters warmed up together, with father Richard tutoring Serena as he talked on a cell phone. Just before taking the court, Venus tended to Serena's dress and made sure it looked right.

"We're really close," Serena said. "I just count on her for so much. I remember one time -- this was so long ago -- I didn't have any money to eat my lunch. Venus went without lunch just to make sure I ate.

"She still does things like that for me. So I kind of feel really as if I'm cheating someone."

But for 50 minutes Thursday, Serena put aside those mixed emotions. Gratitude for your sister's lunch money goes only so far.