I agree with Kui, they know have a rivalry, somewhat. Let's be honest, both of Vika's win could, and should have been won by Serena, you can't say that about both of Serena's wins.
Also, it wasn't Vika's grit that got her that second set, it was Serena's nerve's. Just like in the 2012 Wimby final, her opponent stayed consistent and Serena let her nerve's get the best of her. Both middle sets were on Serena's racquet, and that was reflected by how Rena ran through the third set of both matches.
I believe ran out of gas because of the occasion and the pressure she put on herself.