Natasha enjoys the show. She's trained enough that she can respect the skill of another and Sif certainly has skill. Fighting for Natasha is a means to an end, but it's one she's put a lot of work into perfecting and she knows what it takes to get to this level, Asgardian or not.

"Hi." With a smile, Natasha pushes off of the wall and takes a step closer. "I hope you don't mind an audience. My name's Natalie. I'm new."