Serena- she faces an obstacle and it pushes her to be better. Fed on the other hand has no problem with people who can't challenge him but let him face someone who doesn't bend willingly and someone who makes it so he has to fight for the win and u can rest assured he'll fold.
Dude, no one on the women's tour has anything
to hurt Serena with. Her opponents not bending over is therefore inconsequential. It's not hard to come across as resilient, a fighter, etc when deep down inside, you know your opponent can't really push you.