WTA roundup: Kerber wins all-German showdown in Stuttgart

Two-time tourney champion Angelique Kerber knocked off her German countrywoman Andrea Petkovic in straight sets Thursday at the Porsche Tennis Grand Prix in Stuttgart, Germany.

Fifth-seeded Kerber won 6-2, 6-4 to move on to the quarterfinals. She followed up a bye to face Petkovic, a wild-card qualifier who also happens to be her longtime friend and Fed Cup teammate.

"It's never easy to play against someone you really know for such a long time," Kerber said afterward in comments published by WTA's official website. "I mean, like everybody saw that we are really good friends.

"At the end, we just hugged each other. She said congrats to me and the dinner is on her."

Former world No. 1 Victoria Azarenka of Belarus also advanced with a 4-6, 6-3, 6-4 win over fourth-seeded Karolina Pliskova of the Czech Republic, the defending tournament champion. Azarenka next will face eighth-seeded Estonian Anett Kontaveit, who easily dispatched Anastasia Pavlyuchenkova of Russia 6-3, 6-2.

TEB BNP Paribas Istanbul Cup

Marketa Vondrousova of the Czech Republic reached the quarterfinals in her fourth straight tournament by beating Russia's Svetlana Kuznetsova in straight sets during their second-round showdown in Turkey.

Vondrousova won 6-4, 6-3 to avenge a loss against Kuznetsova in the duo's first match two years ago. She jumped to a 5-2 lead in the first set and never looked back.

The Czech teenager next will face Lara Arruabarrena, who surprised top seed Carla Suarez Navarro during an all-Spanish first-round matchup.

Sixth-seeded Petra Martic advanced with a 4-6, 7-5, 6-3 win over Romania's Ana Bogdan. Also earning second-round wins were France's Kristina Mladenovic and Russia's Veronika Kudermetova.

--Field Level Media