Zuko was a firebending master by the time he was eleven. Not like Azula was, all dizzying new moves, and terrifying tricks, but something – deeper. More basic. He knew exactly how to control the fire under his skin and on it. If he couldn’t do the flashiest or strongest moves – well, he had this, at least, an intimacy with fire he wouldn’t trade for anything.
This means when his father presses a flaming hand to his face, Zuko doesn’t stop him, but he doesn’t take it either. He directs the heat of the flames from his face to the rest of his body and subtly into the floor.
When Ozai pulls his hand back, Zuko is unharmed.
Well. Physically.