Back to playing with bare hands against knives in the silat class, and once again -- even though I've mentioned it a time or twelve -- the realization that against an attacker (or defender) who has a clue what s/he is doing with a sharp and pointy thing in your direction? You are in trouble.
Run away if you can.
If you can't run away and you don't have an equalizing weapon, the functional situation you will find yourself in is what we call Oh Shit! Mode. There are things you can do, though you really don't want to do them unless there is no other choice. All those fancy disarms you see in the movies, those multiple passes back and forth? Good luck with those.
Trading a punch for a stab? Only if you are the stabber.
A guy puts a blade out and has any skill, you go charging in at the wrong moment, you get to be shish kebab, a graduate of Skewer U ...