STUTTGART, Germany - Caroline Wozniacki got some revenge at the Porsche Tennis Grand Prix on Thursday, beating German No.1 and her conqueror in Miami, Andrea Petkovic, in the quarterfinals of the Premier stop.

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A few weeks ago Petkovic became the first German player to defeat a reigning No.1 in almost 12 years, not since Steffi Graf's dramatic victory over Martina Hingis in the finals of the French Open that year; and early on it looked as though Petkovic would repeat the feat, as she burst out to a 4-1, double-break lead, even holding four points for 5-1. Wozniacki just wouldn't let go, however.

The Dane saved those to close to 2-4, another four game points to close to 3-4, and another two game points to close to 4-all. Once she was even she never fell behind again, running the streak to 10 games in a row and building a 64 50 lead. Petkovic squeaked a service hold for 64 51 but Wozniacki capped it, 64 61.

"To win those games in a row from 4-1 down was great for me and I started playing some great tennis," Wozniacki said afterwards. "Andrea was playing well and I was making a few errors, but I just kept doing what I knew I needed to do, stuck to my gameplan, and everything just worked out for me today."

Agnieszka Radwanska, who was one spot out of the seedings this week, followed her good friend Wozniacki into the semis with a hard-fought 75 63 win over another German, Kristina Barrois. Barrois, playing in her first Premier quarterfinal, pushed Radwanska to the limits in the one-hour, 17-minute first set; she led 3-1 the second set when Radwanska won five games in a row.

"It's always tough to play here, not just because it's my first tournament on clay but also the draw is always really strong," Radwanska said. "I'm happy with my performance. Kristina was tough on clay, slicing and hitting lots of topspin - I was in trouble in both sets. I just tried to serve well and be aggressive."

Radwanska, who is now into her first WTA semifinal since San Diego last summer, was asked what she would be doing with her day off on Good Friday. "I think everything's going to be closed so there won't be much to do," the Polish No.1 responded. "I'm afraid I'm going to be stuck in my room!"

Wozniacki leads Radwanska in their head-to-head series, 3-1. This will be their first meeting on clay courts (or on any surface other than hardcourt).

Earlier in the day, No.5 seed Samantha Stosur and another unseeded player, Julia Goerges, reached the final four (read more about that here).