Brother John, a novel by Rutledge Etheridge. First published in 2001. The ISFDB listing calls it #3 in Etheridge's "Duelist" series because it was the last of three volumes to be written and published, but I should mention that it is actually a prequel to the first book, in which Brother John is already a very experienced Duelist who is a close friend of the main viewpoint character.
So Brother John is basically an origin story for its title character, filling in gaps in the reader's understanding regarding how he got started as a Duelist, long before anything that happened in the first volume (Legend of the Duelist). This volume is divided into two portions; the first part happens when John Biggle, sometimes called "Big John," is just fourteen years old. As it happens, he is already bigger and stronger and can move his hands faster than most grown men, so he's already getting a reputation as a very capable brawler. Repeatedly, the narrative text calls him such things as "the young giant."
You mentioned elite professional fighters. They are called the Duelists. They spend a lot of time engaging in prearranged prize fights according a strict set of rules, with professional referees who make sure these rules are enforced. Some of those fights are advertised as "death matches," which apparently means that both sides have agreed, presumably of their own free will, that this fight won't be declared over until someone is dead. (Frankly, that part of the Duelist universe always bothered me a lot, ever since I first read the first volume, sometime in the late 1990s, I think.)
John is tricked into signing a contract to have a prize fight (but not a death match) with a Duelist whose full name is spelled out on the paperwork as "Thomas Klaus Herdtmacher." There's a local resident in terrible mental condition who is called "Cyclone Tom," and John thinks that "Thomas Klaus Herdtmacher" must be the full legal name of that guy. Which is reassuring, because Cyclone Tom may have been a serious Duelist once, but now he's in pitiful condition (too many blows to the head, John figures), and John knows he can beat the guy. This will be of great benefit to his own living relatives -- two young aunts, and his grandfather. He is willing to do a great deal to help his family. (The problem is that they don't feel the same way about him.)
Then the real Thomas Klaus Herdtmacher shows up to meet the young man whom he has agreed to fight (this is quite some time before the scheduled match), and he is disappointed to learn that, contrary to what he was promised, this opponent (John) is only a very big, strong, stubborn, and brave fourteen-year-old with nothing resembling serious professional training in any form of armed or unarmed personal combat. They spar a bit, informally, and John comes to realize that a top-notch Duelist in perfect physical condition is a much, much tougher proposition than anyone he ever scuffled with before. (In other words, Herdtmacher proves he can mop the floor with John any time he cares to take the trouble.)
John, unlike me, was very surprised when, on the night of the prize fight, Herdtmacher "lost," as you described. It turned out that one of his personal secrets was that he had double-jointed knees, and thus he could take a nasty fall and then rise to his feet and give the referee and the audience the strong impression that his leg must be badly broken because of the way the lower portion of it was bent away from the knee at a very unhealthy-looking angle. I didn't know exactly how it would happen, but I had already figured out that this was one occasion when Herdtmacher, with thousands of previous victories under his belt, didn't want to win. Partially because he liked John and felt sorry for him and didn't really feel it would be sporting to beat him to a pulp; partially because he was offended that the promoter who set up the fight had lied through his teeth about what sort of fight it would be. If the promoter then lost his shirt when he had to pay off on all the bets people had made on John at long odds, Herdtmacher wouldn't exactly be crying himself to sleep about it. (He points out that his contract guaranteed him a fixed fee no matter who won, so he wasn't losing any money on the deal.)
As this portion of the story ends, John, disgusted by the lack of gratitude from his blood relatives after he won a bunch of money for them, finally decides to stop calling himself "John Biggle" (his birth name) or "Big John," and just go with "Brother John" instead. That's a decision that will last for the rest of his life (as seen in the first Duelist book).
In the second part of the book, Brother John is five years older, ready to study to be a duelist (because of the positive impression which Herdtmacher made upon him), and is just now arriving at the planet where tens of thousands of young people who desperately want to be duelists come to be tested all at once, and to compete against one another, to see if they are worthy of receiving offers to enroll in one or another of the twelve official schools for aspiring Duelists. As you said, Brother John does great. As the book ends, he has accepted an invitation to join the student body of what is considered to be the truly elite school; the Barrow School, first and best of them all. (Herdtmacher, of course, was a graduate of that school.)