Many tennis players train for additional strength in order to improve their performance.
Strength is needed and it plays a very important role.
But the main physical qualities needed by tennis players are speed and quickness. For example, it is the speed of the arms coming through holding a racquet that determines how fast the racquet is traveling and how hard the ball is going to be hit. Quickness refers more to the ability to change directions while in motion or when getting started from the ready position. Speed and quickness cannot be developed by themselves. Even though they are separate qualities, they are directly related to each other and to other physical qualities.