Holder Marion Bartoli advanced to the quarter-finals of the Stanford Classic on Thursday with a 6-3, 6-4 win over former world number one Ana Ivanovic, while Maria Sharapova also advanced to the last eight.
Russian Sharapova, a three-times Grand Slam winner, beat Belarus's Olga Govortsova 6-3, 6-3 while Maria Kirilenko defeated sixth seed Shahar Peer 6-4, 6-3.
American teenager Melanie Oudin lost 6-3, 6-1 to eighth seed Victoria Azarenka.
Sharapova dictated the action off the ground against the erratic Govortsova, and served and returned well enough to control most of the points.
"I stayed aggressive and made sure to have an extra something on my ball," said the fifth-seeded Sharapova, who will face second seed Elena Dementieva of Russia in the quarter-finals.
"I thought I had the advantage when I hit a good solid serve and then tried to take the ball and do something with it."
Frenchwoman Bartoli, who beat Venus Williams to win the 2009 Stanford title, served and returned aggressively against Ivanovic.
"I'm really pleased I was finally able to get past her," said Bartoli, who had lost all three of her previous meetings against the Serbian.
"I'm moving a lot better so I'm able to get the ball back with interest when she hits the ball hard. I managed to play the big points well and that's the type of matches I need this summer."
Bartoli will face Azarenka, who believes she can regain a place among the world's top 10.
"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.
Peer had beaten Kirilenko in New Zealand at the start of the season but this time the Russian was in control.
"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."
Kirilenko will meet third seed Agnieszka Radwanska of Poland in the quarter-finals, while top seed Samantha Stosur of Australia will play Yanina Wickmayer of Belgium.