>> Head shot.
Yeah, right... melons are tiny things - too hard to hit. Try two closely spaced, aimed shots to the center of mass. (And no, TV watchers, I did not say “double tap”.)
If the knife guy is moving forward, move to the outside of his knife hand as you draw your weapon, and if need be move left and down to the ground with feet between your trunk and the knife as you draw your weapon.