Do you think you have a better chance of winning if you focus on winning, thereby putting pressure on yourself for results, or by focusing on playing well, which maximizes your chances of actually winning? The question answers itself.
You'll always play better and have a better end result if you focus on performance (i.e. playing well) rather than the end result. Performance in this case means all aspects of the game, including strokes, footwork, serve & receive, tactics, and the mental game.
So how do you maximize performance, thereby maximizing your results? By practicing your techniques until they are second-nature, even in a big pressure match, and then approaching those matches where you don't increase the pressure by pressuring yourself to have good results.
In other words, try to have fun when you play so there's less pressure, since it's that self-made pressure that causes one's game to fall apart. Practice your techniques, focus on performing well, have fun, and the results will eventually take care of themselves.
Having fun doesn't mean laughing out loud. Have fun on the inside from the satisfaction of working hard and doing what you have practiced. If you focus on winning, you'll just put pressure on yourself and get nervous. Focus on performance, and smile on the inside.