Clijsters won the title in Stanford last week

The top seed smashed a racket after dropping serve in the eighth game but recovered to win 7-5 6-2.

Clijsters will next face Czech teenager Nicole Vaidisova, who beat Russian Anna Chakvetadze 3-6 6-4 6-3.

The other semi-final will see second seed Maria Sharapova take on in-form Patty Schnyder.

Sharapova beat defending champion Mary Pierce 6-2 6-3 in her quarter-final, while Schnyder upset fourth seed Elena Dementieva 6-4 6-3.

Clijsters admitted that an on-going back problem may have contributed to her outburst against Hingis.

Overall, I'm not satisfied or happy, but maybe it was a bit too soon to return Mary Pierce

"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-fina,l" said Clijsters.

"I was struggling in the first set and the frustration boiled over."

Hingis said that after losing a tight 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 practise after a long layoff following Wimbledon. I hope to be ready for the US Open."

Pierce was playing in her first tournament for six moths after tearing ligaments in her foot and found Sharapova too much to handle.

"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.