Three To See, Madrid: May 11

(1) Serena Williams vs. (7) Sara Errani
Head-to-head: Williams leads 4-0

Sara cites Serena as the toughest opponent she’s ever faced, writing on her WTA blog this week: “[Serena’s] so strong, stronger than the other players.”

Williams relied on her defiance, declarative serve, and timely shotmaking to overcome 45 unforced errors in a sloppy, 6-3, 0-6, 7-5 comeback win over 63rd-ranked Spaniard Anabel Medina Garrigues, who missed a wide open forehand pass that would have given her a match point on Serena’s serve at 4-5 in the decider.

French Open finalist Errani has been sharp, sweeping left-handers Varvara Lepchenko and Ekaterina Makarova in succession. Errani will want to extend rallies, put plenty of balls back in play, and hope Serena's footwork stagnates.

Errani has been spinning her serve in as a point-starter (she served 82 percent against Lepchenko and 80 percent versus Makarova) so Serena should step into the court, take the ball on the rise, and rip high-percentage returns into the corners—and right back at Errani on occasion—to take charge early. Serena spaced out against Medina Garrigues, looking sluggish and hitting flat-footed at times, but she must be on her toes and quick off the mark against the speedy Errani.

The defending champion is much more dangerous off both serve and return, and if she’s patient working her backhand cross-court to the 5'4" Italian’s backhand, she will open up the court for her backhand down the line. Serena typically turns it up near the finish line.

The Pick: Williams in two sets

(16) Ana Ivanovic vs. (2) Maria Sharapova
Head-to-head: Sharapova leads 6-2

The current and former French Open champion face off in a glamour match of former world No. 1s willing to hug the baseline and sting the sidelines. This is a rematch of last month’s Stuttgart quarterfinals, where Sharapova squeezed out a 7-5, 4-6, 6-4 victory en route to her 29th career title.

Ivanovic has impressed with how hard she’s competing against Top 10 opponents. Ana will want to make a high percentage of first serves and make this match about her forehand. If Ivanovic is landing her flat forehand, she can force Sharapova to defend. The 2008 French Open champion is sound closing at net, is 8-1 on clay this season, and should carry confidence from her 6-3, 6-1 quarterfinal thrashing of No. 6 Angelique Kerber.

Numbers don’t always tell the story with Sharapova—she can litter up the stat sheet with errors and double faults and still win—but she’s riding a 20-match clay-court win streak and should be highly motivated by milestones looming this weekend. Sharapova is one win away from her 500th career victory and 50th career final, and if she betters Williams by one victory here, she will surpass Serena and regain the world No. 1 ranking.

Since Ivanovic thumped Sharapova, 6-2, 6-1, in the 2007 French Open semifinals, Maria has beaten Ana five times in a row, winning nine of 10 sets in that span. The resourceful Sharapova finds a way to win matches—she owns a 17-2 record in three-setters over the past two seasons—and while I think this could be an entertaining battle, I see Sharapova advancing to her fourth consecutive final.

The Pick: Sharapova in three sets

(5) Rafael Nadal vs. (WC) Pablo Andujar
Head-to-head: Nadal leads 1-0

If I told you before the tournament began none of the top four seeds would reach the semifinals and one of the final four would be a 113th-ranked wild card, you might have thought the odds of that occurrence would rival the chances of Madrid returning to its blue period.

Yet Andujar has taken down 10th-seeded Marin Cilic, big-serving John Isner, and 14th-seeded Kei Nishikori—who defeated defending champion Roger Federer—to emerge as a surprise semifinalist. Andujar has been willing to grind: Five of his nine wins on the season have come from a set down.

Two-time Madrid champion Nadal was nearly sent packing by David Ferrer, who was two points from a straight-sets upset, only to see Nadal storm back for a 4-6, 7-6 (3), 6-0 comeback. Rafa left his backhand short in the court against Ferrer and paid the price for two sets. Andujar has to hope the faster track in Madrid will help his cause and he has to attack any mid-court balls he sees.

Riding a 29-2 record into the semifinals, Nadal is the overwhelming favorite to win his fifth title of the season. He is too experienced—and too familiar with the sting of upsets (the loss to Lukas Rosol at Wimbledon and his three-set defeat to 73rd-ranked Horacio Zeballos in the Vina del Mar final in February)—to look past Andujar and make a misstep.

The Pick: Nadal in two sets