In a duel, man to man, sword against sword, it can be a lack of skill that gets you killed. Often as not, though, it`ll be a matter of luck, or if it goes on too long, then it`ll be the man who tires first that tends to die.
In the end it`s about staying power. They should put that on headstones, "Got tired." Maybe not tired of life, but at least too tired to hold on to it.
In a real fight, and most fights are real, not the artifice of a formal duel, it`s fatigue that`s the big killer. A sword is a heavy chunk of iron. You swing that around for a few minutes and your arms start to get ideas of their own about what they can and can`t do. Even when your life depends on it.