My sister was playing in the 18 and under division of the Quicksilver National Open in Southern California. I was 11 years old and watching my sister play. I don't remember the outcome of her match, but I do recall an incident two courts down that will never leave my memory. There was an argument (more like a screaming match) over who won the first set. You see, the second set concluded, and one of the players was walking to the net to shake his opponent's hand. But the opponent was ready to play a third set. Someone cheated, big time! It was an early round, so they didn't have a chair umpire to keep track of the score and watch the lines.
Unless you're playing a well-funded tournament, there are no referees to chair your match. So, you must keep track of your own score, be in agreement with your opponent, and make line calls of your opponent's shots while trusting that your opponent will make fair calls as well. From my experience, the score rarely gets mixed up and, when it does, someone is only confused by a few points. If the players can't resolve the score, they go back to the point they both agree on. The story I opened with, rarely happens. However, bad line calls and mind games take place at every tournament and nearly every match.
A bad line call is when your opponent calls the ball "out" when your shot lands "in." They're stealing a point from you. This usually happens during pressure moments. For example, it's your serve, 5-5 in games, and 30-30 in the game; you serve a massive ace that touches the line. Everyone saw it "in," but your opponent calls it out. Now, you must hit the second serve at 30-30 even though it should be 40-30, a game point for you. You're dealing with score pressure, the annoyance from your opponent cheating, and the despair from not serving with a lead at 40-30 (a different form of score pressure). They also diminished your great first serve, the big ace. You might find yourself taking less risk moving forward. How do you handle this situation?
You can question the call, and let them know you know. Or, you could pretend like it's nothing and assume you missed. In the latter case, ask yourself, do you want your opponent to get away with taking a point from you? Probably not. So, you must respond but do it in a way that doesn’t hinder your state of mind — you must remain focused.
Kids who intentionally make bad calls usually do it because of pressure to win from parents or coaches. Sometimes it comes from within, they want to win no matter the cost. Other players make bad calls to get into your head. They principally don’t care about the point, but use it for strategic weakening — it bothers you, so your mind is thinking about the bad calls and not on playing.
You must accept cheating is part of the game. The reasoning here is you can't control what other players do or how they act. So, you don't want someone else getting into your head and affecting your game. You should thus find peace with the notion that sometimes your adversary won't be fair. This way, you are free to play without getting frustrated by their hooking. You should still apply pressure on your opponent after they make a bad line call. You can call them out on it and ask for a roving referee to watch your match.