Pennetta was runner-up in Los Angeles last year
Tenth seed Flavia Pennetta brought Maria Sharapova's run to an end at the LA Championships with a three-set win in Saturday's semi-finals.
The Italian won 6-2 4-6 6-3 after fighting back in the final set as Sharapova struggled on serve.
Australian Sam Stosur made the final after powering past Sorana Cirstea of Romania 6-3 6-2.
Stosur will appear in her fifth career singles final on Sunday as she tries to land her first singles title.
"My whole career I've been trying to get to this point," said Stosur, who was widely regarded as a doubles specialist before a superb recent run of form.
"It kind of looks like I've done it late, but I don't worry too much about that. I took a little longer to develop."
I feel physically I'm getting better and getting a good sense of the court, but it's still a work in progress
Sharapova had been hoping to win her first title since returning to the tour in May after a 10-month lay-off following shoulder surgery, but there were further signs that her remodelled service action is still a problem.
The Russian, wearing tape on her right shoulder for the first time this week, was wildly erratic in her semi-final, making 61 unforced errors and 16 double faults as she let a 3-1 lead slip in the deciding set, and losing her final three service games.
"I didn't feel fresh," said Sharapova. "I was surprised it went to the third and I had my opportunities, but I couldn't close it out."
She added: "It's almost like new territory for me. With every tournament I feel physically I'm getting better and getting a good sense of the court, but it's still a work in progress."
Pennetta, who was runner-up in Los Angeles last year, said: "I was 3-1 down and I was thinking it was over, but I just tried to fight.
"It was a little bit difficult for me. I started to feel a little pressure. She's a great champion, it was not easy to beat her."
She added: "The serve was a problem for both. I was very nervous with my serve because she was returning so well. She started to make a lot of mistakes."