Usually is when she plays hootie. Serena's footwork and balance are pretty bad out there. Not in position lining up her shots and going for winners from impossible angles. Off balance. Serving is poor too. Hootie is playing well though.
Serena will want this to go 3 at least. She should fight tooth and nail to keep her winning record against Azarenka in slams. It only gets harder once they break that barrier.