Victoria Azarenka overcame a slow start to upset defending champion Marion Bartoli 3-6, 6-3, 6-3 in the quarter-finals of the Stanford Classic on Friday.
The Belarussian was joined in the last four by top seed Samantha Stosur, who battled to a 7-5, 3-6, 6-3 win over Yanina Wickmayer, Russian fifth seed Maria Sharapova and Poland's Agnieszka Radwanska.
Sharapova ended second seeded compatriot Elena Dementieva's comeback tournament with a marathon 6-4, 2-6, 6-3 victory in a late match that lasted two hours and 47 minutes.
She advances to face third seed Radwanska, who eased past Sharapova's friend Maria Kirilenko 7-5, 6-0.
While the fourth seeded Bartoli is ranked higher than the eighth-seeded Azarenka, the 20-year-old Belarussian had beaten the Frenchwoman in their three previous meetings so the outcome was not a surprise.
Bartoli got off to a terrific start, powering her groundstrokes to the corners to forge a 6-3, 3-1 lead and appeared well set for a routine victory.
However, after Azarenka fought hard to hold serve in the next game, the tide turned completely and she reeled off seven straight games with a forceful attack.
Bartoli tried to regain control late in the third set but Azarenka mixed deft drop shots with laser-precision backhands and broke her opponent to win the match on a forehand error.
Later, Bartoli told reporters that Azarenka's loud grunts had distracted her.
"It's difficult to play against those kinds of players," Bartoli said.
"It's fine to grunt sometimes when you make an effort, but sometimes it's just so loud.
"It's hard to focus on the other side of the net. But it's not something I can get bothered by, because otherwise I would lose my concentration so much.
"I just need to forget about it, but it's hard," she added
Azarenka said that she had been grunting since she began to play tennis and was not the only player who could be distracting, indicating that Bartoli's preparations to return serve were also problematic.
"A lot of things can distract people," Azarenka said.
"When someone is (shaking her racket up and down) when you are serving can also be distracting.
"I don't pay attention. That's the way I play and I cannot change it," she added.
Azarenka will next face Stosur, who had to come back from an early break in the third set to repel the athletic Wickmayer.
"It was tough from the first game to the end," Stosur said.
"I got a little flat in the second set.
"I was down 2-0 down in the third and I finally started to pick it up and stayed away from playing into her patterns," she added.
Sharapova was given a stern lecture by coach Michael Joyce after losing the second set and immediately benefited from a riskier brand of tennis in the decider.
She broke for a 5-3 lead when the second seed, playing her first tournament since she was sidelined by a calf injury at the French Open early last month, doubled-faulted and won the match with a screaming forehand winner.
"In the third set I just had to put more balls in play and I think she was surprised that I managed to keep them in the court," Sharapova said."I knew I had to pick it up, I kept trying to do the right things and eventually it paid off," she added.