against an unskilled opponent, it's hard enough, especially as the opponent will often move unpredictably and attack you "wrong", LOL, but if you have been well-trained to deal with your own adrenaline response, you stand at least a chance if you use everything available to you (environmental layout, improvised weapons, etc.) in a tactically organized manner; and we have all seen video evidence of people stabbed multiple times by enraged, yet tactically disorganized assailants who have survived; so again, randomness seems to be the watchword - as a trained fighter, your goal is to limit that randomness and try to take as much control of the situation overall as possible

now, take that tactical mindset and give it to your knife wielding opponent as well: well, you're basically fu(ked, because unlike the amateur knife wielding psycho, he'll go for target selection, and will set you up to get it (e.g. - distracting you with his empty hand with the knife as the hidden, follow-up attack); I guess that the answer is to carry your own blade (or similar ranged weapon such as a telescopic baton) and wear body armor at all times (or at least leather wrist bands, gloves and jacket - good leather can be effective turning a blade to varying degrees, at least more than a t-shirt); of course, this is if you live the kind of life where this sort of scenario is a concern to you - if it's not, the one advantage is that the chances of you randomly coming up against a trained knife fighter are probably pretty slim to none...