When you are competing in tennis, it is easy to get wrapped up in the match. However, when you lose a particular game, set, or maybe even a string of matches in a short amount of time, you could easily get discouraged and assume that just because you have lost before, you are bound to lose again. It could be just as easy to become overconfident after winning a single game or set decisively and assume that just because it was so easy to win that portion of the match, you are undoubtedly going to win the entire match. However, in the sport of tennis, players are forced to approach each game, each set, and each match with a clean slate.
In tennis, the score does not carry over from game to game or set to set. This provides the athlete with a unique opportunity to reset himself or herself constantly throughout the match.  Therefore, it is vital that you have some way of assuring yourself that the match is not over after a single good or bad game, or even a good or bad set. Say for instance that you win a game, and it was completely one-sided. Or, you had a slow start and lost the first set. That does not mean that the entire match is over. There is a reason that you get to keep playing. As long as the competition is still going, you still have an opportunity to win. Refuse to allow yourself to be defined by a single point in the match.
It is essential to have some means of reminding yourself that there is still more tennis to play regardless of the circumstances. For example, maybe you tap your racquet after you won a game decisively to remind you not to be overconfident for the next game. Or maybe, you lose the first set, but bounce the ball a couple of times before serving to remind yourself that you still have the opportunity to win and play well despite losing that first set. Regardless, approach every game, set, and match as a new opportunity to win!