samatha meditation... or zhine...
google it, or look for some budhists or zen people, they can teach you. its a simple technique, but that doesnt mean its easy.
we are used to respond to every stimuli around, and this teaches you to concentrate in one thing only, and to observe your emotional and thinking patterns...
it will help your tennis and your overall life, having less stress and a stronger immune system for example, and you will deal in a whole different way with your Unforced errors
withtime and practice...