Nobody ever has defeated Maria Sharapova in Stuttgart, although plenty of notable champions have tried.  Sharapova has claimed titles in both of her previous appearances at the Porsche Grand Prix, conquering rivals like Victoria Azarenka, Li Na, and Petra Kvitova in the process.  But her perfect record in Stuttgart will be put to the test when the two-time defending champion faces top seed Agnieszka Radwanska in a Friday quarterfinal.

Extended to nearly three and a half hours in the first round, Sharapova rallied from within two points of defeat to battle past Lucie Safarova.  She advanced more smoothly on Thursday, avenging a February loss to compatriot Anastasia Pavlyuchenkova in straight sets.  Handed a first-round bye as the top seed, Radwanska dropped just five games in the second round to clay specialist Roberta Vinci.  Although Vinci has struggled mightily in 2014, that result may build Radwanska's confidence considering her past aversion to this surface.

Whereas the Pole has produced her least convincing tennis on clay, Sharapova ranks among the most dominant clay players of her generation.  The Russian won Roland Garros two years ago and reached a second straight final there last year.  She has not lost to anyone other than Serena Williams on red clay since 2011, and she has won both of her previous clay matches against Radwanska.  In fact, Sharapova has thoroughly dominated her rivalry with the Pole, winning eight of their 10 meetings and seven of their last eight.

These trends suggest that Sharapova should enter their quarterfinal as the favorite despite her lower ranking.  Still, Radwanska can take comfort in knowing that the indoor red clay of Stuttgart sometimes plays more like a hard court than the outdoor clay tournaments that have frustrated her.  Although she owns just two victories over Sharapova, those victories came in two of their most important meetings.  Radwanska prevailed on the grand stage of the 2007 US Open and halted Sharapova two years ago in the final of Miami, a Premier Mandatory event.

Both Radwanska and Sharapova have produced some solid results on paper this year, such as an Indian Wells runner-up effort for the Pole and a Miami semifinal run for the Russian.  Each also has suffered some puzzling losses, however, to opponents from Pavlyuchenkova and Camila Giorgi (Sharapova) to Bethanie Mattek-Sands, Flavia Pennetta, and Alize Cornet (Radwanska).  Both were convincingly outhit at the Australian Open by Dominika Cibulkova, much less renowned than either of them.

Neither Sharapova nor Radwanska has won a title in 2014, but the winner of their quarterfinal might be favored to claim Stuttgart.  Sharapova must topple Radwanska to preserve her place in the top 10, at least for a few more weeks.  She always has placed a greater emphasis on titles than rankings, though, so the prize of a third consecutive Porsche probably inspires her more.  

The contrast of styles between Sharapova's ferocious aggression and Radwanska's deft counterpunching should offer plenty of compelling tennis at the Porsche Grand Prix on Friday.