2 Drills For Developing Powerful Tennis Serves from youtube by Feel Tennis Instruction
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The main reason why your tennis serve lacks power is because you keep trying to hit the service box. Because the service box is rather difficult to hit when we add some power, we'll try to aim and "steer" the ball into the service box.
That of course holds us back and we never can develop full power - we simply develop a habit of serving with a lot of control. That's why it's critical that when you're performing tennis serve drills or work on your technique, that you also hit many balls freely without aiming into the service box.
The video above shows two drills you can apply to develop more power in your tennis serves.