Usually sunny Kim Clijsters smashed a racket in a moment of first set frustration on Friday before settling down to a straight-set win over Martina Hingis and a semi-final berth in the 1.34 million-dollar WTA tournament.
The sweet-faced Belgian top seed let her emotions rip after Hingis broke back for 4-all in the first set, but channelled her frustrations into a 7-5, 6-2 victory over the eighth-seeded Swiss.
And once she had extended her 2006 quarter-final dominance over Hingis, who returned to the game in January after three years away, Clijsters managed to explain away the out-of-character episode.
"I've had some injury problems and I don't feel that my game's quite there. But it's great to get to another semi-final," said the winner of last weekend's event in Stanford, northern California.
"It was tough tonight," added the reigning US Open champion. "I was struggling in the first set and the frustration boiled over."
Hingis, ranked 13th in the world, said that after losing a combatative opening set that featured five breaks of serve, her resistance cracked.
"When you lose a 5-4 lead and probably the next eight points, it's like a blackout," said the Swiss. "The match was very intense, very mentally tough."
"She puts such pressure on you, she's always on top of you. She's hitting winners off of first serves and had no weaknesses. What I need is more match practice after a long layoff following Wimbledon."
"I hope to be ready for the US Open," Hingis said.
Clijsters has now beaten Hingis three times this year - all at the quarter-finals stage - in Melbourne, Paris and San Diego.
She saved seven of nine break points while breaking Hingis five times.
Clijsters next faces Czech teenager Nicole Vaidisova, who will reach a career-first top 10 ranking on Monday after posting a 3-6, 6-4, 6-3 victory over Russian Anna Chakvetadze.
Second-seeded Russian Maria Sharapova mowed down defending champion Mary Pierce 6-2, 6-3, winning six consecutive games en route to a semi-final clash with Patty Schnyder.
Sharapova needed just 58 minutes to book her meeting with Schnyder, who reached her second semi-final in as many weeks with a 6-4, 6-3 victory over Elena Dementieva.
Schnyder, runner-up last weekend at Stanford to Clijsters, pounced on more than 40 unforced errors from Dementieva, one of three Russians who reached the last eight.
Sharapova took control against sixth-seeded Pierce at 2-2 in the opening set, reeling off six games on the trot.
The Russian was never seriously threatened by Pierce, whose movement was hindered by heavy taping on each thigh.
"My legs have been sore for the past few days and they were getting worse and worse," said Pierce. "The trainers advised me to have them taped to prevent any further injury."
"Overall, I'm not satisfied or happy, but maybe it was a bit too soon to return," added the 31-year-old, who was playing her first event after a six month layoff nursing a tendon injury in her right foot.
"I'm just glad I was able to compete. Today against Maria was a good test. I wasn't at my best and I couldn't move well around the court. But to play the match that I did was encouraging."
Sharapova now holds a 3-1 head-to-head lead over Pierce, her only loss coming at the 2004 US Open.
"In the first set I got up and early break, but we had to feel each other out," Sharapova said. "I got my rhythm and lifted my game.
"I'm excited to be in the semi-finals, I've played three good matches so far."