STUTTGART (AFP) - Germany's Australian Open champion Angelique Kerber withstood a tough opening second-round match against compatriot Annika Beck to reach the quarter-finals of the Stuttgart WTA indoor tournament on Wednesday.

Kerber, the defending champion on clay in Stuttgart, needed just over two hours to seal her 4-6, 6-3, 6-1 win to reach the last eight after receiving a first-round bye.

The pair of Germans last met in the fourth round of the Australian Open when Kerber beat Beck in straight sets on her way to the title.

"It's always difficult to play against another German, who you know so well," said Kerber, who played alongside Beck in Germany's Fed Cup play-off win over Romania last weekend.

"I am now just happy to have gone further in the tournament." Kerber's win marked a good day for Germany with both Andrea Petkovic and Julia Goerges also winning their first-round matches.

Earlier, 2015 US Open finalist Roberta Vinci served out a tense first-round encounter with former top-10 player Ekaterina Makarova by dispatching the Russian 7-6 (7/3), 4-6, 6-4.

In the second round, Italy's Vinci will play 2011 Stuttgart champion Goerges, who outfoxed France's Alize Cornet 6-4, 6-0 in just one hour and 16 minutes.

Petkovic had few problems as she won her fourth straight encounter against France's Kristina Mladenovic for a 6-2, 6-4 first-round victory.

Petkovic will next take on top seed Agnieszka Radwanska on Thursday in the second round.

Third seed Garbina Muguruza of Spain also made it to the quarter-finals with her 6-2, 6-2 win over Timea Babos of Hungary.

The only shock of the day came when eighth seed Lucie Safarova of the Czech Republic crashed out 7-6 in a hard-fought 7-6 (7/4), 4-6, 7-5 win for compatriot Karolina Pliskova.

Pliskova now faces former world number one Ana Ivanovic of Serbia in the second round on Thursday.