1. Tennis is about being relaxed and confidently going for your shots. Relaxed confidence is your best weapon.
2. Tennis is about "controlled aggression" - controlled with your strokes, and aggressive with your feet - or put another way, controlled with your goals, aggressive about getting there.
3. Tennis is about execution - hitting through the line of the shot, and leverage - letting the racket head do the work.
4. In tennis, the most important point is the next one.
5. Tennis is a game of percentages. (There are no guarantees, only probabilities.) Thus, follow the directionals almost all the time, and manage your errors.
Post Script: It's a bit amazing, but if you substitute the word "life" for the word "tennis" in the above 5 points, it all still seems to ring true! Yes, tennis can be a metaphor for life.