after watching this zoomed in a bunch of times Rock really is leading with his face towards Smith's hand and reacts as he swings but it's also possible he just figured out what Smith had in mind because they are friends or something and he read it in his face.
I have been (deservedly) hit by friends and known as the person was approaching me what their intent was.
He definitely looks ready for it though I agree with you on that part!
there are some hints!