The main concern in tennis is getting the ball over the net and into your opponent’s court. As you’ll gather soon, modern tennis is a rotational rather than a linear game for the body as well.
Gravity is the main force in nature. Release a ball from your hand and it goes down. Therefore to clear the net you need to LIFT the ball. Tennis is more of a vertical game than a horizontal game.
Air resistance slows down the ball considerably and it curves it downward naturally into the opponent’s court. Further, forward ball rotation, called “topspin” in tennis, pushes the ball down even faster, adding impressively to the safety factor of the hardest shots you hit. Rather than extending the racket through the hitting zone* as in baseball, you brush the ball across the strings, feeling your power accomplish several things: ball speed and rotation, and a kicking ball hard to control.