Best thing to do is cower in fear and make it look like you're going to hand ovrr your wallet, then their guard will drop and they'll move into your fighting arc, whereupon you can nut them, give them four stiff fingers in the throat, etc, and take their wallet instead. I mean, run away.
There was an amusing incident about 20 years ago. A lad pulled out a knife to rob someone. Unfortunately, his victim had seen Crocodile Dundee and whipped out a panga and robbed the original robber, who then had the cheek to go and report the while thing to the dibble. Once we'd finished wiping away our tears we we happy to put handcuffs on himm if only for his own safety.