IMHO if you find yourself in the situatiion that you are facing a determined, well-trained knife fighter, even with a gun you can die before you clear your holster.
Personaly, IF I have a chance to see the knife and am lucky enough to get my weapon clear, I will yell out for them to drop the knife as loud as I can (if there is time) and if he advances to within lethal range with no intention of stopping, double tap him in the chest and call your lawyer, then 911.
