By Steven Wine
The Associated Press

KEY BISCAYNE, Fla. - Venus Williams doesn't lose often, and she doesn't lose easily. With a gallant last-ditch rally, Williams erased eight match points, but she couldn't sustain the comeback and lost to fellow American Meghann Shaughnessy 7-6 (2), 6-1 Monday night at the Nasdaq-100 Open.

The fourth-round upset spoiled a potential quarterfinal showdown between Williams, a three-time champion, and No. 6 Jennifer Capriati on Wednesday. Advancing instead was Shaughnessy, who is seeded 23rd and beat Williams for only the second time in their seven meetings.

"I'm so happy," Shaughnessy said. "It's not even going to sink in for a little while."

The 5-foot-11 Virginian fell behind 3-0 and then began to dominate, whipping shots into the corners that put Williams on the defensive. Shaughnessy won the final four points in the tiebreaker, then raced to a 5-0 lead in the second set before Williams dug in. She saved four match points in the sixth game, then four more in the next game.

"You don't want to know what was going through my mind," Shaughnessy said with a laugh. "I definitely took too long to finish it there. I was a little nervous and didn't go for a couple of shots."

Williams made forehand errors on the final three points, slapping a shot 6 feet long on the ninth match point. Shaughnessy thrust both fists toward the sky, then gleefully walked to the net, where Williams greeted her with a smile and a handshake.

"I always felt like I could do it until I ran out of time," Williams said. "It's just back to the drawing board."

Her sister, top-seeded Serena Williams, advanced by beating Iroda Tulyaganova 6-0, 6-4. Her opponent in the quarterfinals will be 18-year-old Marion Bartoli, who summed up the challenge of playing the defending champion in charming fractured English:

"She win everybody," Bartoli said.

That's why the path to a title is about to become a whole lot tougher for Bartoli, who has led a charmed life since arriving at Key Biscayne.

After winning two qualifying matches, Bartoli drew a wild-card entrant, a lucky loser and a qualifier in the first three rounds - and beat them all. She won again Monday when seventh-seeded Lindsay Davenport strained her right hamstring in the second game and was forced to retire after losing the first set 6-0.

Serena Williams is unbeaten in 14 matches this year.

"It's very exciting," Bartoli said. "Nothing to lose, just to enjoy to play against the No. 1 in the world."

Other quarterfinal matchups include No. 3 Kim Clijsters against No. 9 Jelena Dokic, and No. 4 Justine Henin-Hardenne against No. 12 Chanda Rubin.

Rubin beat No. 8 Amelie Mauresmo 6-0, 6-2, and Capriati ended a strong run by American compatriot Sarah Taylor, winning 6-1, 6-0. Both matches took less than an hour.

In men's play, five-time champion Andre Agassi routed No. 32 Jarkko Nieminen 6-2, 6-0 in 58 minutes. Agassi's opponent in the fourth round Tuesday will be wild-card Mark Philippoussis, who edged Thomas Enqvist 6-7 (6), 7-6 (3), 6-3.

No. 4 Roger Federer, who lost to Agassi in last year's final, beat Juan Ignacio Chela 6-1, 3-6, 6-1.

Under constant pressure from Shaughnessy, Venus Williams' shotmaking turned shaky, and she committed 32 unforced errors to go with some wild serves. She had six double faults, including one to lose the tiebreaker, and Shaughnessy won 21 of 29 points against Williams' second serve.

"It's impossible to win every match," Williams said. "I couldn't seem to get the ball to go in today always."

By the second set, Williams was so flummoxed that Shaughnessy had her chasing the ball across the net. Shaughnessy hit a backhand crosscourt volley with so much backspin that it bounced back over her side of the net, leaving Williams with no chance to reach it.

"I'm not going to sit here and tell you that I practice that shot," Shaughnessy said with a grin. "It was my night tonight."