MANHATTAN BEACH, California (Ticker) -- Serena Williams' 21-match winning streak came to a stunning end Friday.

The 12th-seeded Chanda Rubin shocked the world No. 1, recording a 6-2, 4-6, 7-5 triumph to reach the semifinals of the $585,000 JP Morgan Chase Open hardcourt event.

Second seed and world No. 3 Jennifer Capriati also went down to a surprising defeat at the the hands of unseeded Ai Sugiyama of Japan, 6-3, 6-3. But defending champion Lindsay Davenport avoided another upset with a 7-6 (7-2), 6-2 victory over Greece's Eleni Daniilidou.

This marks just the fourth time in the 27-year history of the Los Angeles event that neither of the top two seeds will appear in the final.

Rubin will face fourth seed Jelena Dokic in Saturday's semifinals after the Yugoslavian eased past Italy's Rita Grande, 6-4, 6-1.

During her run, Williams captured the Rome, French Open and Wimbledon titles, dropping just three sets along the way.

The 20-year-old Williams, who also captured titles at Scottsdale and Miami earlier this year, had not been beaten since falling to Belgium's Justine Henin in the final of the Berlin claycourt event.

"This isn't a setback; I'm still No. 1 ," Williams said. "It was definitely a bit frustrating, but you have to give her all the credit. She definitely played a good match. But I never felt my rhythm. I didn't play very well at all today."

Williams won here in 1999 and 2000 but squandered seven match points in a quarterfinal loss to Monica Seles a year ago.

"All week, even in practice, I haven't been where I want to be. So it's difficult to go out and have a lot of confidence in your match," Williams said. "I don't think my practices were up to par. I was trying, I was fighting. But no matter what I did, I couldn't get the rhythm in practice. I think it subconsciously transferred to my match."

Rubin avenged a straight-sets loss to Williams in the round of 16 at the All-England Club.

"I wanted to be aggressive when I had my opportunities. I needed to serve well and, off the first serve, I needed to be quick off the ball. I knew that coming into the match," Rubin said.

After taking just 23 minutes to secure the opening set, Rubin held a 4-3 edge in the second. But Williams reeled off seven of the next eight games to capture the second set and open a 4-1 cushion in the third.

"She started playing better and starting hitting the shots, crazy shots with pace," Rubin said. "I was kicking myself for a while in the third set. But it was just a question of getting back to a game plan and fighting through to the end of the match."

Rubin rebounded to knot the decisive set at 5-5, broke Williams in the 11th game, then served out the match.

"It's definitely so far the biggest victory I've had, with her being No. 1 and being an on-form player, coming off two Grand Slam wins. For sure, the biggest win to date," Rubin said.

Meanwhile, Capriati was never in her 56-minute match and lost for the third time in five career meetings with Sugiyama.

"I just came out flat and couldn't get it going," Capriati said. "I was missing a lot of easy shots, shots that I hadn't been missing. I definitely wasn't there today, not moving nearly as well as I have been. Everybody has their off days."

Capriati outlasted 16th seed Tamarine Tanasugarn of Thailand on Thursday night. But completing the three-set match less than 24 hours before her quarterfinal may have taken too much out of Capriati.

"I'm sure it had something to do with it," she said.

Capriati does not believe, however, this stumble will disrupt her preparations for the U.S. Open.

"I got to just keep going forward," she said. "It's just everyone is going to come out and play their best against you because they have nothing to lose."

Appearing in just her second semifinal of the year, Sugiyama will face the third-seeded Davenport.

Daniilidou was seeking her third straight upset after knocking off sixth seed Daniela Hantuchova and No. 10 Anne Kremer. But Davenport wasn't about to join Williams and Capriati on the sidelines.

"It's surprising to lose both of them in one day. But upsets have happened before in tournaments," said Davenport, who has reached the semifinals in each of her last three events. "Obviously, I didn't want to keep the trend going. I wanted to do well here."

Dokic is gunning for a third WTA title in 2002 after victories at Sarasota, Florida and Birmingham, England.

The top prize is $93,000.