The pro's have very highly-developed motor programs, unlike many of us, so they don't have to think about *how* they hit the ball.
They just watch the ball and use their memory and observational/anticipation to determine where the opponent is likely to hit his/her next shot. They also watch the opponent just before he/she hits the ball. The most important time to watch the ball is just after it bounces on *your* side of the net
They can adjust their strategy and tactics according to the opponent, or according to the score: Play conservative on the big points, gamble a little bit when they have 40-love in their favor, etc.
The best ones know how to manage their emotions and not let the loss of a point cause them to lose concentration.
They spend a lot of time conditioning, because they know that if they are not in tip-top shape, they will falter in a long, difficult match.