Stanford, California :France's Marion Bartoli, Maria Sharapova and Victoria Azarenka of Belarus advanced to the quarterfinals of the Bank of the West Classic on Thursday.
Israel's Shahar Peer, another seeded player, wasn't as fortunate.
The fourth-seeded Bartoli beat former world No. 1 Ana Ivanovic of Serbia 6-3, 6-4, for her eighth consecutive victory at the event and 10th win in her last 13 matches.
"It was a great match," Bartoli said. "It was a good build up for the U.S. Open. It's good to have these kinds of matches."
Bartoli beat Ivanovic for the first time in four career meetings.
"When I played Ana before she was on the rise to becoming the No. 1 player in the world," Bartoli said. "Now I finally passed her. Having a higher ranking than her helped my confidence and gave me the mental strength to play the game."
Ivanovic fell to 3-6 over her last nine matches.
Former world No. 1 Sharapova, the fifth seed, beat Belarus' Olga Govortsova, 6-3, 6-3 and the eighth-seeded Azarenka topped American Melanie Oudin, 6-3, 6-1 in the second round, while Russia's Maria Kirilenko knocked off the sixth-seeded Peer, 6-4, 6-3.
"It was a sloppy first game but the rest of it was fine," Sharapova said. "I took my chances when I could."
The 16th-ranked Peer became the first seeded player to fall at the event, losing for the fourth time in her last six matches after opening the year 18-5, which included a three-set win over Kirilenko in New Zealand.
"This time I decided to be a lot more aggressive," Kirilenko said. "This year has been so much better and given me a lot of confidence."
The 27th-ranked Kirilenko, who meets No. 3 seed Agnieszka Radwanska in Friday's quarterfinals, is assured of her best finish at Stanford in three trips.
"I played not too badly," Kirilenko said. "I can play better."
She's also had her career best results at the Australian Open, when she reached the quarterfinals, the French Open, when she made the fourth round, and at Wimbledon, when she was eliminated in the third round.
Azarenka, who broke into the top 10 last year, reached her sixth quarterfinal of the year by winning her eighth in her last 10 matches.
"I think I played very well," Azarenka said. "It was a little tough at the beginning with a couple of service breaks but I felt strong the whole match. For me it's important to play every point the same, no matter what the score is. Whether I am up or down 40-love, that is always a key for me."
She plays the winner of a late match between defending champion Marion Bartoli of France and former world No. 1 Ana Ivanovic of Serbia.
Oudin also thought she competed well.
"I had a game point almost every single game," she said. "I don't think she overwhelmed me. It was close."