In spite of all of her that's changed, Sif is very much herself in all the ways that matter. Her heart is the same, and, more importantly, her movements across the field of battle are the same. This is the dynamic they've always had: Thor is powerful but Sif is agile.

She darts in and out of his range of motion, and she doesn't hesitate to use his newly-monocular vision to her advantage. She spins and twirls, dodging strikes almost balletically and shifting to his right so she can jab him in his literal blind spot. A cheap shot, to be sure, but this is what he wanted. He has to relearn how to fight with his narrowed range of vision, so she exploits it whenever she sees an opportunity.

That isn't to say her form is perfect. While she hasn't lost her prowess, muscle memory alone isn't enough to replace centuries of training. There are gaps in her defenses, obvious to the experienced eye as mistakes that a young trainee might make. He lands blows to her side, her arm, and she has to remind herself to be more guarded.