After doing a lot of practicing this weekend, I thought I might give my two cents about writing fight scenes, both medieval-style and renaissance-style, when it's a one-on-one scenario between two trained fighters.
What could I possibly know about this? Surprisngly, a lot. I've been doing historical swordplay for awhile now, and have done full-speed full-contact sparring (albeit with blunt steel weapons, not sharps), mainly longsword/bastard sword and rapier. I've also witnessed full-armor sparring, again with blunt steel weapons, but legit kits. This by no means makes me an expert on anything, but it does offer me a few insights.
I'm going to put aside the mechanics of how to describe actions, and balancing technique vs. story, and instead offer some food for thought on a general level.
1) Fighters will have some level of fear going into a combat situation, unless they are actively trying to die. Having someone trying to skewer you on a sharp steel weapon is scary, and historically, even a minor flesh wound could have become infected and caused an unpleasant death. So, for fighters with a wish to stay alive into their old age, survival instincts would dictate a physiological response of fear. How they deal with that fear depends on their personality, their experience, their training, what's at stake, etc etc.
2) Sparring is an adrenaline rush. That physiological response of fear I was talking about? Yeah, it causes adrenaline spikes. That comes with lots of fun effects like not feeling hits as much, getting into the zone, and reacting by instinct more than by thought. A character could conceivably be an adrenaline junkie, and get into fights just for the high it gives, much in the same way some people are into extreme sports. They might also make rash decisions post-fight, when they're still riding the adrenaline and not using the analytic part of their brain.
3) There is a zone for sparring. It's not exactly having a blank mind, but more of an intense awareness of your physical self, and your surroundings. Finding that place is easier or harder for different people, because it's really easy to get distracted by other concerns, or over-thinking the fight. Sure, sizing up the opponent is a great idea, analyzing their movements, all of those good things require thinking, but I like to call it "back of the mind" thinking. While that sort of combat analysis is happening, the blank awareness dominates the main thought processes.
4) Different people have different fighting styles. Even if they've trained under the same master, and know all of the same moves, different people move differently. Some people will use brute strength, some will use perfect technique, some will be quick and agile, some will use feints, some will have an amazing sense of timing and measure. Whether any of these will always win against another, I have no idea, but I do know that it changes the dynamic of the fight a lot.
5) Continuing on the thought of fight dynamics, level of training or experience changes the fight a lot. Someone with little or no training will flail wildly; while this isn't effective, it is difficult to deal with because it is unpredictable, and can result in a lucky hit getting in. Someone with decades of training will probably move with a fluidity others can only envy, and they're challenging because their instincts have been honed far more than their opponents, and their "oh shit!" reactions will be more likely to be the correct reaction than yours.
6) Swords level the playing field. Put a sword in someone's hand, and suddenly the fact that they are a 6'7" muscular giant, or 5'0" and lean makes a whole lot less difference. They both have advantages and disadvantages to their height/weight/muscle to fat ratio, and one will not automatically triumph over the other. It will simply change their reactions to their opponents to best use their own strengths and exploit their opponents weaknesses.
7) Fighting in armor is hard. While it isn't exactly heavy when you put it on, you can feel it weighing on your limbs. I tried on a chainmail shirt, and while it wasn't heavy, it actually felt like it was constricting my lungs, and it became harder to breathe. This means that people in armor will want to finish off their opponents as quickly as possible, with the least amount of energy possible. So, hour-long fights? Not going to happen, unless their armor is enchanted to be light as a feather. It is also damn hard to find the weak spots in armor, which means that there will be a lot of scuffling and hits that have no effect until a weak spot is exploited (armpit, under the helm, etc..). The easiest way to get someone is to knock them off their feet and then finish them off from there. Having a back-up weapon like a dagger is exceptionally useful in this situation.
If I think of anything else, I will add it, but as some final thoughts, if you want to find out what holding a real steel weapon feels like, find your local western martial arts group (if you're in a major city, there's probably one near you), and if that isn't possible, there are a lot of historical manuscripts from the Italian, German, French, English, etc... traditions available online, from various eras, with translations to English done by some dedicated people. Research is your friend!