The coin toss
When I was a teenager, I played a lot of tennis. Just well enough to compete and have fun, but not well enough to take it too seriously.
At that level, there might be an official overseeing the final match of a tournament, but for the most part you called your own points. On the rare clay court you might have the benefit of a ball mark, but more often we played on hard courts and if your opponent called a deep ball out, you had no choice but to take their word.
Plenty of people bent the rules, calling close balls in their favor, but there were also players who blatantly cheated, daring you to get upset about it.
Our high school didn’t have the most robust training program so some of us supplemented team practice with a semi-private clinic. Our teacher was a talented player who could pull off just enough trick shots and dirty jokes to keep a few teenagers engaged for an hour.
He also taught us about the coin toss. When it becomes clear during a match that your opponent isn’t going to call it fairly, walk off the court and tell the tournament official that you won. Doesn’t matter how far into the match you’ve gotten. If you’re not gonna get a fair shake, get off the slow moving bus that’s headed toward a cliff.
Your opponent will have no choice but to dispute the result, and if you hold firm, all the official can do is toss a coin. At that point, your chances are 50/50, but that’s a heck of a lot better than no chance.