MADRID, Spain - Saturday at the Mutua Madrid Open will see all four semifinalists in action. First up is defending champion Serena Williams against Sara Errani before current French Open champion Maria Sharapova takes on one of her predecessors, Ana Ivanovic.
 Serena Williams (USA) vs.  Sara Errani (ITA #7)
Head-to-head: Williams leads 4-0
Sara Errani may have described her as the toughest adversary on tour in her blog this week, but there was a hitherto unforeseen air of vulnerability about Serena Williams' play on Friday. Across three sets against wildcard Anabel Medina Garrigues, 46 unforced errors sprayed from the racquet of the World No.1, who was forced to rely on grit rather than groundstrokes to pull her through in the end.
A similarly errant display in the semifinals against Errani could prove terminal to her title defense; Errani is the WTA's wiliest counterpuncher and with her speed and skill on the dirt is sure to make Williams earn her place in the final.
Yet, despite the Italian's feats on clay over the past two years - five titles, 40 wins and a French Open final - the match-up with Williams is not a favorable one; she has only taken one set in their four career meetings and has never defeated a member of the Top 5, let alone a reigning No.1.
 Maria Sharapova (RUS #2) vs.  Ana Ivanovic (SRB #16)
Head-to-head: Sharapova leads 6-2
There is a lot on the line for Maria Sharapova going into her semifinal with Ana Ivanovic in Madrid. The Russian, who is riding high on a 20-match clay court winning streak, is one win shy of her 500th on tour and is also still in with a shout of dethroning Williams at the top of the rankings.
For Ivanovic, the rewards for victory are less tangible. Over the past year or so, under the tutelage of Nigel Sears, the Serb has slowly been working herself back into the title picture at the game's biggest events, although she is yet to make that final step.
A win over Sharapova would not only be her first over member of the Top 2 since 2007, but also see her return to the final of a top tier event for the first time since Indian Wells in 2009. Achieving that is easier said than done, though; Sharapova has won the pair's last five matches as well as 58 of her last 59 matches against players outside the Top 10.