Italy's Flavia Pennetta stunned former world number one Maria Sharapova 6-2 4-6 6-3 in the semi-finals of the LA Championships on Saturday.
Pennetta, the 10th seed, will meet Samantha Stosur in the final after the Australian used her big serve to overwhelm Romanian teenager Sorana Cirstea 6-3 6-2.
Playing her eighth match in the 12 days, three-times grand slam champion Sharapova clearly lacked energy as the match wore on and squandered a 3-1 lead in the third set thanks to a litany of double faults and sloppy errors.
"I didn't feel fresh," said Sharapova, who double faulted 16 times and had 61 unforced errors compared to 23 from Pennetta.
"I was surprised it went to the third and I had my opportunities, but I couldn't close it out."
Pennetta played an aggressive match, rarely giving ground even when Sharapova tried to force her off court with big groundstrokes.
"To play against her I have to be very aggressive and in the second set I started to wait for her mistakes and that's why the match changed so much," said Pennetta, who also reached the LA final last year before falling to Dinara Safina.
Playing in her sixth tournament since returning to the tour in May following shoulder surgery, the 22-year-old Russian was heartened by her week overall, but knows she has a long way to go before regaining championship form.
"It's almost like new territory for me," said Sharapova, who hasn't won a title since April 2008.
"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." In reaching her first final of the year, Stosur nailed seven aces and won 85 percent of her first serve points.
Cirstea had suffered a right foot injury in her upset of Agnieszka Radwanska on Friday and never looked comfortable moving, committing 25 unforced errors.
