Maria Sharapova's Wimbledon build-up suffered a setback on Saturday when she was knocked out of the Birmingham grasscourt event 6-4, 6-4 by China's Li Na.

Sharapova, the champion here in 2004 and 2005, is still feeling her way back after over 10 months out with a shoulder injury and she showed signs of a quick revival by reaching the French Open quarter-finals.

But the unseeded Russian was plagued by errors on Saturday, suffering six breaks of serve in her 90-minute defeat.

"I definitely wasn't playing with the same intensity I had in the last couple of matches. Some days you have it, some days you don't," said Sharapova who had defeated Li at Roland Garros.

"Against an opponent like her, that intensity has to be there every step of the match. I'm still getting used to a few things in my game. I just have to keep working, keep trying to get everything better and better."

Li, the fourth seed, will face Slovakian 13th seed Magdalena Rybarikova, who beat India's Sania Mirza 3-6, 6-0, 6-3, in Sunday's final.

"My return of serve was so good today. I broke her many times so it did not feel like a grasscourt match," said the Chinese star who had never defeated Sharapova in five previous matches.

"I lost to her so many times before, so I thought, if I lose again then I should not worry. That made me very relaxed today.

"I was up 4-2 in the second set and double-faulted twice. I was up 4-2 in the third set last week too. When I double faulted, I was thinking, 'What are you doing?'"

Rybarikova won a streakier affair with Mirza, dropping the first set but taking the second set at love and extending her streak to eight games in a row by building a 2-0 lead in the third.

Mirza tied things up, 2-2, and held even for 3-3, but the wheels fell off again as Rybarikova won 12 of the last 15 points of the match. "She played aggressively from the start and it surprised me," Rybarikova said.

"I had to start playing aggressive myself. I think my serve was the key to the match. Grass is my favourite surface - I reached the junior final of Wimbledon and I like playing at the net. I'm hoping I can continue this in the final."

It will be Rybarikova's first final, her best results before this week being semi-finals at Tashkent last year and at Hobart and Pattaya City earlier this year.

Li has been to five singles finals previously, winning Guangzhou in 2004 and Gold Coast in 2008 and finishing runner-up three times, including Monterrey earlier this year.