World No. 1 star Victoria Azarenka was tested mightily in her quarterfinal victory Friday at the star-studded $740,000 Porsche Tennis Grand Prix.

The Australian Open champion went the distance to stave off German crowd favorite Mona Barthel 6-4, 6-7 (3-7), 7-5 on the indoor red clay at Porsche Arena. The Belarusian needed 2 hours, 48 minutes to survive a sloppy match that featured 24 double faults, including a whopping 16 by Barthel. The young German also uncorked 13 aces, but had her big serve broken seven times.

The rising Barthel captured her first-ever WTA title in Hobart, Australia in January.

Azarenka is now a brilliant 28-1 this season, including a perfect 4-0 record against her next opponent, Agnieszka Radwanska.

A fourth-seeded Radwanska continued her recent winning ways with a 3-6, 6-2, 6-3 come-from-behind victory over eighth-seeded French Open champ Li Na of China.

The world No. 4 Radwanska has won seven straight and 17 of her last 18 matches, including a big victory over Maria Sharapova in a final in Miami last month, as well as a championship in Dubai in February.

The 2012 Stuttgart winner will claim $115,000 and a new Porsche sports car.