The problem with that is after the punch lands, you can't undue the damage to respond. Just in the last year or so there have been several people killed or permanently injured by a sucker punch.
If someone hit you and then stopped, and was not about to do so again, no justification. Call police. But if you are in the midst of an assault, and don't have the ability to end it post haste [with fisticuffs] most people would be in reasonable fear of serious injury or death... which meets the threshold, legally speaking.