Miami: Top seed Wozniacki upset by German Petkovic

World number one Caroline Wozniacki suffered a surprise fourth-round defeat by Germany's Andrea Petkovic in the Sony Ericsson Open on Monday, falling victim to a smart strategy from her lower-ranked opponent.

Petkovic, ranked 23rd, played positive attacking tennis in the Florida sunshine at Key Biscayne to move into the quarter-finals with a 7-5, 3-6, 6-3 victory over the Dane.

Petkovic saved 12 of 17 break points against Wozniacki but the foundations of her success were an aggressive serve and subtle variations in her game.

She resisted the temptation to try and slug it out with Wozniacki, opting instead to mix up her shots and employ a range of different speeds, then celebrating her victory with a jig she has dubbed the 'Petko dance'.

"Most of the players think they can overpower Caroline. I think that's the wrong approach, because that's where she's most comfortable, when she can run and bring the most balls back," Petkovic told reporters.

"What I try to do is mix it up and to make her play, and then when I had the short ball to go for it."

Wozniacki lost the opening set after squandering three set points and later said she had only herself to blame.

"I had so many chances in my first set ... Then, just the energy level dropped a little bit and I lost the match. That's what happens in sport," she said.

Russian Maria Sharapova, still working her way back from injury, eased past Australian Samantha Stosur, ranked fifth in the world, winning 6-4 6-1. Sharapova, currently ranked 16th, has never been beaten by Stosur in seven meetings.

Second seed Kim Clijsters's match with Serbian Ana Ivanovic was postponed to Tuesday due to heavy rain.