MADRID - Serena Williams overcame a lethargic start to upset fellow former world number one Caroline Wozniacki at the Madrid Open on Thursday and set up a quarter-final meeting with old foe Maria Sharapova.

The ninth seed powered to a 1-6 6-3 6-2 victory over the Dane, clubbing a blistering 43 winners to reach the last eight in the Spanish capital for the first time.

"I had a really slow start today, I don't know why," Williams told a news conference. "I was just a little sluggish, mentally I was fighting some demons, maybe.

"The first set was not me at all. I haven't been playing like that and I just thought I got do to better than this and I started just to try to play my game."

Wozniacki, who beat Williams for the first time in four attempts at the Sony Ericsson Open in Miami earlier this year, started strongly but lost control of the match in the second set.

The American broke Wozniacki's first service game in the third set and, with the sliding roof open over the sun-drenched Manolo Santana center court, tightened her grip.

The 2009 runner-up in Madrid saved a first match point with a fizzing crosscourt shot but was helpless at the second as Williams wrong-footed her with another powerful drive.

The controversial, blue clay surface, however, did not seem to be a problem for Williams.

"This clay is a little slippery," she added. "I feel a little bit like an ice-skater out here, but actually I can ice-skate so it's not a bad thing. I think I can play on any court."

Caroline Wozniacki of Denmark returns the ball to Serena Williams of the US during their women's singles match at the Madrid Open tennis tournament, May 10, 2012. [Photo/Agencies]

Upper hand  

Williams will meet second seed Sharapova, another former number one, who progressed without hitting a shot in the morning after unseeded Czech Lucie Safarova pulled out with a gastro-intestinal illness.

Current number one Victoria Azarenka of Belarus, losing finalist in 2011, plays China's Li Na on Friday after both went through to the last eight on Wednesday.

"She is doing really well," Williams said of Sharapova, whom she has beaten in their last six meetings.

"She does have the upper hand going into tomorrow's match, she has the momentum after winning (in Stuttgart) last month. So I'm just going out there with nothing to lose."

Serena Williams of the US returns the ball to Caroline Wozniacki of Denmark during their women's singles match at the Madrid Open tennis tournament, May 10, 2012.[Photo/Agencies]

Wozniacki, still seeking her first title in 2012, had few complaints at the result.

"Williams is a great opponent but to be honest I am just happy that my foot's feeling better, you know I hurt it in the first match," she said.

"I think I can take a lot of confidence from this match leading into the French Open, which is coming up soon."

The winner between Williams and Sharapova will meet either fifth seed Samantha Stosur or Czech qualifier Lucie Hradecka in the semi-finals.

The 26-year-old Hradecka, who stunned third seed Petra Kvitova in the second round, beat Russia's Ekaterina Makarova 6-2 7-6 to reach the last eight.

Fourth-seeded Pole Agnieszka Radwanska, who climbed to world number three this week, won 7-6 6-4 against Italy's Roberta Vinci and will meet American qualifier Varvara Lepchenko, a 6-1 6-7 6-3 victor over Spain's Anabel Medina Garrigues.