Sharapova said her hectic playing schedule was partly to blame
Maria Sharapova's unbeaten start to the season came to an end as she was beaten 6-3 5-7 6-2 by fellow Russian Svetlana Kuznetsova at the Pacific Life Open.
Australian Open champion Sharapova had won 18 straight matches before running into Kuznetsova, who will meet top seed Ana Ivanovic in Sunday's final.
Ivanovic edged past fellow Serb Jelena Jankovic 7-6 (7-3) 6-3.
Meanwhile, unseeded Mardy Fish shocked seventh seed David Nalbandian to set up a last-four meeting with Roger Federer.
The American continued his unbeaten record against the Argentine with a 6-3 6-7 (5-7) 7-6 (7-4) victory.
It's pretty much been non-stop with all the tournaments I've been playing... It takes a toll on your body and your mind as well
Federer received a walkover into the semi-finals after Germany's Tommy Haas withdrew from their quarter-final with a sinus infection.
The world number one is attempting to reach his first final of 2008.
Haas, 29, had beaten sixth seed Andy Roddick and Andy Murray, seeded 11th, on his way to the quarter-finals.
"I am very sorry for Tommy as I was looking forward to another great match with him," said Federer, who has won eight of 10 meetings with Haas.
"We have played quite a few times, and I know and respect Tommy a lot. I hope he gets better quickly."
In the other semi, Rafael Nadal faces Novak Djokovic.
Sharapova was outplayed when it counted by second seed Kuznetsova in a fluctuating match lasting two hours and 19 minutes.
"There were a couple of important points that could have changed the match and she ended up winning them," said the 20-year-old.
"I think from that she gained confidence and kind of steamrolled after that in the third set.
"She's a very tough, tough player. She's not number two in the world for nothing. I give her a lot of respect for her consistency, what she's able to do on the court, how strong she is and how many balls she gets back."
Sharapova added that she was feeling the effects of a hectic start to the year.
"I'm playing a lot of tennis, been flying a lot," she said.
"It's pretty much been non-stop with all the tournaments I've been playing, and Fed Cup as well. It takes a toll on your body and your mind as well.
"You feel like you have to go out on the court and spend a lot of emotion and energy out there, and sometimes you just don't have it for every match."
Kuznetsova said: "It feels great. I don't think Maria played like always she can play. Neither did I. But I think it was a very good, interesting third set."