Top seed Kim Clijsters overpowered Martina Hingis 7-5, 6-2 to reach the Acura Classic semi-finals on Friday in San Diego.
Second seed Maria Sharapova crushed defending champion Mary Pierce 6-2, 6-3, setting up a last four clash with Patty Schnyder of Switzerland, who beat number four Elena Dementieva 6-4, 6-3.
Belgian Clijsters will face seventh seed Nicole Vaidisova of Czech Republic after she beat Russian Anna Chakvetadze 3-6, 6-4, 6-3.
Hingis came back from a 2-4 deficit in the first set to take a 5-4 lead, but Clijsters served brilliantly and came back to win the set 7-5.
A deflated Hingis looked sloppy in the second set and Clijsters pounded out the win.
"Mentally she puts so much pressure on you," Hingis said. "She doesn't really give you a free point. She doesn't really have a weakness."
Former world number one Hingis, who returned to the tour in January after more than three years off due to injury, said her game was not yet at a standard where she could compete with the elite.
LACKING STAMINA
"I'm lacking stamina," the 25-year-old said. "It's the focus, I wish I could play her every day so I could get used to her intensity.
"I'm pushing the top players, but you have to play matches like today a lot. I'm not happy with just reaching the quarter-finals."
In other quarter-finals, Sharapova forced the pace against a sluggish defending champion Pierce, dictating with a big serve, powerful ground strokes and sporadic charges to the net.
"You don't know how you are going to play after a long break and I played three opponents with three different games and I've come out and feeling solid and healthy," said Sharapova, who has won only one title this year.
"I've been working on things, like coming in and seeing those opportunities. I've been stupid the last few times so I'm learning my lesson.
"No matter how good or bad I feel, I still need to go into net."
