Getting sucker punched means that you weren't ready, which means there is no defense for it.
If you were ready for it, it wouldn't be a sucker punch
That being said, I wouldn't even bother using the fist. If you can get an open hand to his face, that means you can also grab his head, which means you could also slam his head into the nearest wall.
I know this is tangential but FWIW:
this is exactly where the body skills mentioned in the Jo-Trick Forum come into play.
Once you train your body to move in a more efficient manner in on a day to day basis in regular movement, coupled with the way the body is physically conditioned in oft neglected solo training exercises(which are the heart and core of any decent system), when the #"$" hits the fan, it can only move in a certain manner in response to a stimulus, only this time there's no techniques jamming your cerebellum. Jus some food for thought.