There are three main principles of the Inner Game – quieting the mind, non-judgment and trusting the body. The player must first learn to apply them in training. And when he experiences these effects in training he knows what to look for in the match. He then plays the inner game – how to perform at his peak.
These are the fundaments of tennis psychology and by understanding these principles and applying them daily a player develops mental toughness. He knows how to apply the tools and as a very positive side effect gains a lot of self confidence.
He now knows that he has ways of dealing with his own mind and that most of his competitors do not. He is aware that he has the edge in the most stressful situations in the match and that gives him self confidence.
By constant application of these tools a player becomes more and more proficient with them and finds his ideal mental state very quickly if he happens to lose it. These losses of ideal state happen less and less often.
The above summary comes from an article on TennisMindGame.com on how to develop a still mind and enter the zone. Here’s the full article.