Marianne Werdel Witmeyer's final shot trickled over the net and toppled Arantxa Sanchez Vicario.
No. 1-ranked Sanchez Vicario made the earliest departure by a top-seeded player in the 11-year history of the Lipton Championships, losing, 6-2, 7-5, in Monday's third round.
She will remain No. 1, even if second-ranked Steffi Graf wins the tournament.
Werdel Witmeyer, a hard-hitting Californian ranked 33rd, earned the biggest victory of her career and established a trend that the men's field followed for much of the day.
Third-seeded Michael Chang, No. 4 Alberto Berasategui and No. 7 Jim Courier also lost. Top-ranked Pete Sampras and No. 2 Andre Agassi withstood the wave of upsets with straight-set victories.
Werdel Witmeyer upset Gabriela Sabatini at this year's Australian Open and reached the semifinals, where she lost to Sanchez Vicario.
This time, Werdel Witmeyer controlled the match from the start. She won three service games at love in the second set and broke Sanchez Vicario in the final game.
On match point, the former Stanford player hit a backhand off the net cord. Sanchez Vicario never moved as she watched the ball fall softly on her side of the court.
"I would have rather not won match point on a let cord," said Werdel Witmeyer, "but take them any way you can get them."
Werdel Witmeyer wears sunglasses when she plays, but not because she's famous. Her next computer ranking--no worse than 27th--will be the highest of her career, at age 27.
"I guess if you look at the average age, I am definitely over the hill," she said, "but I don't feel like it."
Her husband, Ron Witmeyer, is a minor league first baseman in the Oakland Athletics' system who declined a request to become a replacement player during the baseball strike.
"Hopefully this will bring some good luck into the family," she said.
Sanchez Vicario, the Lipton champion in 1992 and 1993, won eight titles last year but has none this year through three tournaments. She declined to blame windy weather, her sore ankle or bad luck for the most recent defeat.
"I am not disappointed," she said. "There are many more tournaments to play. You cannot win everything; that is the way I see it."
Courier will support that view. He had beaten Mark Woodforde three times in the past 10 weeks without dropping a set, yet lost to the Australian on Monday, 6-3, 6-4.
"I guess we are getting a bit sick of each other," said Woodforde, ranked 34th.
Berasategui lost to Jaime Yzaga of Peru, 6-2, 4-6, 6-3, and Dutchman Jan Siemerink beat Chang, 6-4, 3-6, 6-2.
Siemerink, ranked 51st, has a good chance to advance. No seeded players are left in his quarter of the bracket.
Sampras, a two-time defending champion, beat Jared Palmer, 6-4, 6-4, and Agassi swept Tommy Ho, 6-0, 6-2. No. 8 Wayne Ferreira eliminated 18-year-old Mark Philippoussis, 6-4, 7-5, and No. 10 Andrei Medvedev beat Patrick McEnroe, 6-4, 7-6 (8-6).
On the women's side, third-seeded Jana Novotna beat Dominique Monami, 7-6 (7-1), 6-2; sixth-seeded Natasha Zvereva defeated Karina Habsudova, 7-5, 6-2; and Lisa Raymond beat No. 16 Zina Garrison Jackson, 6-2, 1-6, 6-1.