Serena Williams  vs Sara Errani . For the third round in a row, Serena lost the first set and had to make a dramatic comeback to keep her hopes of winning her 3rd French Open alive. She had nearly twice as many unforced errors in her 1-6 7-5 6-3 win in the 4th round over Sloane Stephens. Happily, she will play Sara Errani whom she has only lost one set against in 7 matches, but Errani did win a non-tour match on clay in Federation Cup competition during the Spring on home turf in Italy. Errani has reached the French Open final herself and is averaging an astonishing 15 unforced errors per match in the first four rounds, including a mere 3 unforced errors in her fourth round win (forty less than Serena at the same stage of the tournament). Regardless, this tournament is clearly now Serena's to lose and I find it impossible to believe that Sara Errani is going to be the player to prevent that from happening. Mad Professah's Pick: Williams.
Timea Bacsinszky 
Ana Ivanovic  vs Elina Svitolina 
Garbine Muguruza  vs Lucie Safarova