Serena had problems in last 3 matches. Only easy win against Hlavackova, and with Friedsam, Azarenka and Stephens all 3 matches in 3 sets. With Azarenka is ok, but with Friedsam I was watch that match. Serena played very bad that match.
Errani cannot play against Serena because she has very weak service, and every servis-game for Errani is break-chance for Serena.
I think we will in this match 1/4 finals of Roland Garros see a very easy and routine victory by Serena because it's time for one good match from her, and Errani is good oponent.