Azarenka, Sharapova sneak into Stuttgart semis

The world's top-two women's tennis stars, Victoria Azarenka and Maria Sharapova, survived their quarterfinal matches Friday at the star-studded $740,000 Porsche Tennis Grand Prix.

The top-ranked Australian Open champion Azarenka 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 prevail in 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 back in January.

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

The second-seeded former No. 1 Sharapova had to come from behind to best fifth-seeded U.S. Open champion Samantha Stosur 6-7 (5-7), 7-6 (7-5), 7-5 in a war of attrition that lasted just over three hours. The tall Russian smacked 10 aces and had her serve broken only once by the ultra-fit Aussie Stosur.

Sharapova is a three-time Grand Slam champion who finished as the runner-up to Azarenka at this year's Aussie Open and Petra Kvitova at last year's Wimbledon extravaganza.

A fourth-seeded Radwanska, meanwhile, 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 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.