The first key difference between the kick serve and the flat serve is the location of the toss. The mechanics of the toss for either serve are the same: hold the tennis ball in your fingers (but not your finger tips or your palm), raise your arm in a smooth and relaxed motion, release the tennis ball at about the top of your head and continue to raise your tossing arm after you’ve release the ball. The location of where you toss the tennis ball is what’s different. When hitting a kick serve, you want to toss the tennis ball so that you can hit it when it’s directly above your head. A good way to get the tennis ball to the right location is to aim for your hitting arm shoulder when you toss. For example, I would toss the tennis ball up over my head and try and land it on my left shoulder (because I’m left handed I toss with my right hand – for all your right handers out there it’s the opposite). Again, that should put the tennis ball in the right spot – directly above your head – when you hit it.
Now I compare my kick serve and my flat serve side-by-side to illustrate how the location of my toss differs depending on which serve I’m hitting. In these shots, I’m hitting to the ad court. The serve to my left is my kick serve and the one to my right is my flat serve. As I wind up on my flat serve, I toss the tennis ball into the court so that I can hit it a little bit in front of my body. However, on the kick serve I toss the tennis ball so that it drifts over my head. That’s because, later on in the serve, I’m going to make contact with the tennis ball in a different spot than I will with the flat serve. That spot – my point of contact – is directly above my head.
You have some flexibility in terms of how far you toss the tennis ball into the court. That flexibility depends on how aggressive you are trying to be with your kick serve. In the video, I’m not being particularly aggressive so my toss stays more or less in line with the baseline. However, when Frank hits his kick serve, his toss is drifting into the court because he’s being more aggressive with his kick. He’s trying to hit it harder (with more pace) than I am. Tossing the tennis ball into the court like Frank is doing is fine as long as your body follows your toss into the court and you get yourself under the ball at contact. Even though Frank has tossed the ball into the court, his body position is the same as mine at contact.