One of the greatest aspects of the game of tennis is that there is always a chance to win.
I have played tennis for over 50 years and have experienced this on both sides. I have been on the verge of winning and ended up losing the match (many times) and on the brink of defeat and pulling it out (also many times).
In my coaching career, my goal is to make the player feel that they can go out on the court and win the match. It is one thing to say this, but a totally different thing to actually believe it. The best way to prove the idea to yourself is to pay attention to the good points that you play. Even in matches that end up in a lopsided game score, there are likely points that you play well and chances to win games. Make sure you remind yourself of these good points and try to repeat what made them so good.
Also, remember that the two biggest enemies keeping you from this "belief" are panic and fear. If something bad happens, human nature tends to take over resulting in panic which turns into the feeling of being afraid you are going to lose. The best players are able to resist the tendency to drift towards these two enemies, and often play a great point after a previous disappointing one.
Going back to "playing every point the best that you can" will help you overcome the perceived failures (which they are not) and increase your ability to believe that you can win, regardless of the situation.
Steve Annacone, USPTA Elite Pro, is the Director of Annacone Tennis (http://www.annaconetennis.com) and MyHamptonsPro based in Sag Harbor, NY (http://www. myhamptonspro.com) and Tennis Professional at Ventana Canyon Golf and Racquet Club in Tucson, AZ. For details on lessons, clinics, or coaching, contact Steve at email@example.com or call 865-300-7323