Thor deserves to fight someone who's known him long enough to see through his moves, to challenge him with pure Asgardian ferocity and determination. Sif should be that person, but she isn't. Not really, without her memories to inform her of his strengths and weaknesses. She can only guess at his skill.

That doesn't mean she won't fight him, though.

"What say you?" she calls, approaching him from across the field. "My blade against the weapon of your choice."