The best fighting technique is the survivor technique. A knight, if he is over 20 years old, has shown his art of surviving many times. The modern master of the sword NEVER survived in the conditions of sword fighting, corny there is no possibility. Secondly, if both participants are of the same age, then the knight learned from childhood, and the master of the sword, at best, from adolescence. The third is fencing technique. The knight's style is pure utilitarianism (hit and hit hard), while modern swordsmanship is overloaded with unnecessary complexity. And the tasks of the knight and the swordsman are different (because the art of swordsmanship appeared only in the late Middle Ages)
thus on the side of the knight experience, training and determination, on the side of the swordsman, technique and experience of generations. difficult to choose a winner
The question does not take into account a lot of nuances, which is why a reasonable answer is, in principle, impossible. For example - in what technique and with what weapon does a modern master fight? A lot depends on this.
Therefore, by my own arbitrariness, I introduce additional conditions.
The master and the knight possess one technique and one weapon. They fight. And before the fight, either both do not wear armor, or both do. In the first case, the knight loses nothing. He is taught from childhood to fight this way and that. But in the second case, the master can lose a lot if he does not feel like a fish in water in armor.
Let's go further: the conditions of the duel. If you are fighting for points with a bunch of restrictions, then it is unrealistic to predict the result. Although, most likely, the master has more chances (I don't think that the knights were engaged in such garbage as a duel "for points").
But if the fight is to death, the master can safely order a coffin for himself. After all, a knight is a professional assassin. This is a special attitude to the fight, taught to short from childhood and honed regularly in real, not training "meat grinders".
The Master simply cannot cope with the pressure of a warrior who is used to - and loves - to kill.
An important point: everything said in the last paragraph (about fighting to death) was confirmed to me by a person who has been fond of historical fencing for many years - in knightly armor and on real swords.
True, he is made a couple of additions:
1) real knights had competitions that were essentially close to our battles "for points" (so the chances of a modern master are also reduced here).
2) to align the initial chances of victory for both participants would have to:
A) either the medieval knight should be seriously fattened and treated (including from worms, vitamin deficiency, etc.). Otherwise, he will come out to a duel weakened in comparison with a modern master. The fact is that in the Middle Ages, nutrition - especially healthy, and even more so, fortified and high-calorie - was bad. Even the gentlemen of the knights. Plus + a bunch of chronic diseases in almost everyone. After all, medicine, in comparison with modern medicine, was generally - in a deep ... cave ...
B) either send a modern master to the Middle Ages before a fight, so that he weakened from poor nutrition, picked up sores and weakened. Thus, he became a knight on a par with the Middle Ages.
The best swordsman. Against a good swordsman with a sword, a knight in armor and with a sword is practically powerless, simply due to less mobility. The skater stabs the "armored" enemy in the slit of the armor while he is making a swing. Similar victories were described in fiction and historical literature, in fact, the very transition from armor and a heavy sword to much lighter swords and sabers (and the abandonment of armor armor) was simply due to their higher efficiency.