Newton's 1st Law of Motion. (law of inertia)
Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. In tennis, the ball doesn't move until it is put in motion for the game, by hitting it with the racket.
Newton's 2nd Law of Motion (force = mass X acceleration)
The Second Law of Motion states that if an unbalanced force acts on a body, that body will experience acceleration ( or deceleration), that is, a change of speed. The force the racket imparts to the ball, changes it's acceleration.
Newton's 3rd Law of motion
For every action there is an equal and opposite reaction. Have you ever hit a ball on a hard ground and it bounced back up to you, well that's example of Newton's 3rd law. Newton's 1st law defines that reacts when the ball bounces over the net in tennis. Another way it happens is by the racket hitting the ball to go in opposite direction.