When I learned how to play tennis, I remember being told to keep a firm grip on the racquet to help eliminate wrist movement during the swing. Obviously, tennis has changed pretty dramatically since then and almost every player relies on some wrist action to add speed to their shots.
Starting with a very loose grip in the ready position will allow you to use your wrist to speed up the racquet just prior to contact with the ball. Your grip will naturally firm up as the ball and the racquet meet, but your wrist will have already increased the speed of your racquet resulting in more speed and spin on your shot.
Keep in mind that the longer you are on the court, the more the tendency is to tighten up your grip before the point has even started. Try emphasizing the relaxed grip before and after each shot. This will help your body naturally incorporate some wrist action into the swing, adding racquet head speed, more miles per hour, and more rotation on your shots.