In Summary My Comments/Suggestions are:

• If I answer this question fully I'm giving away the secrets of a lifetime and I have spent a lifetime in tennis coaching.
• It's pretty obvious that when you hit the ball trying an overhead service you are hitting the ball in an upward direction.
• If you were to hit it parallel to the court at a sensible height the ball would naturally fall (because of gravity) but not before it reaches the net if you hit it with reasonable power.
• So... don't hit it upward from where you're standing - hit it parallel to the court surface and it will fall into the court.
• Try it, but don't try to hit it at full stretch. Hit it at a comfortable height from your body.
• If you want to learn the correct service technique watch our Service Coaching Videos
• Best of luck to you.