In tennis it is very important to have Rhythm and timing, too many people want to hit the ball hard straight away but this is pointless if you don’t have good timing.
It is better if you work on your rhythm and timing in order to get the right feel of the ball coming off your racquet feeling effortless.
When you feel the ball you will start to get your timing right, then you can add a faster swing that will create more power and a heavy ball which will be harder to hit back.
When practicing you should be watching the ball on to the strings of your racquet and contacting the ball out in front of your body and in or near the middle of the strings of your racquet. You should also breathe out a little longer as you hit the ball to relax the body to get good co-ordination of your body. Keeping practicing and then as your timing improve increase your racquet speed.