If you feel you are being cheated and you can keep focused and still play good tennis then that is the best solution.
Sometimes this is very difficult to do so here is a step-by-step way to help with the problem.
The Limit Five Solution
1. First bad call - have a good look and maybe raise your eyebrow.
2. Second bad call - ask if your opponent is certain of the call.
3. If you think you have received a third bad call - tell the opponent in a calm voice, "this is the third bad call. I hope we do not have a problem here. I am positive the ball was in and if you have any doubt we must play a let".
4. If your opponent refuses to budge on the call then call the referee, explain calmly the actions you have taken and ask them to provide an umpire as you no longer have any confidence in your opponent’s ability to be fair. If you remain calm and rational you will impress the referee and possibly intimidate the opponent.
5. If the referee cannot supply an umpire or watch himself then when he leaves you explain the following to your opponent. 'You have chosen to play by your rules rather than tennis rules. Is this your final choice or do we go back to tennis rules? Their response will probably be something like you do not know what you are talking about or simply to shut up. Wait for them to stop and then simply say this. Either you stick to tennis rules now or I will call by your rules as well; 9 times out of 10 this taking charge of the situation in a calm way will solve the problem.
However if this does not stop them cheating, you must make a personal choice of how you will make calls for the rest of the match. The alternative is to disrupt the match every bad call or close call by calling the referee each and every time they make a call remotely close, thus forcing the referee to take action as the game is being disrupted so much.
Whatever course of action you take - remember your most powerful tool is to remain calm at all times.