The second key difference between the slice serve and the flat serve is the direction you swing and pronate when you hit the tennis ball. On a flat serve you come straight through the tennis ball and slap it – hit it clean – but on a slice serve you want to come across the ball and generate side spin that will cause the ball to hook and skid when it bounces.
In the video above, my flat serve is on the right and my slice serve is on the left (I’m serving to the ad court). When I get to the “L position” the video freezes and you can see that my racket is point in different directions. On the flat serve, my tennis racket is pretty much point at the ad court because, when I hit, I’m going to swing straight through the tennis ball. In other words, I’m swinging in the same direction the ball goes. However, on the slice serve it looks like my racket is facing more toward the deuce court because I’m going to swing across the tennis ball to generate side spin.
When we play the videos forward from the “L position” and freeze them at contact, the other key thing to note is that despite the fact that I’m swinging in different directions, my tennis racket strings are still facing the same direction at contact. What generates side spin on the tennis ball is the direction my strings move across the ball at contact, NOT the direction my strings are facing at contact.
Playing these clips forward one more time from the “L position,” what I’ve done to get my strings facing the same direction at contact on both serves is time my pronation a little bit differently on my slice serve.
The final thing I want to do is clarify the relationship between your toss location and your swing direction. Why do I want to put the tennis ball further to my side when I’m hitting a slice serve than when I hit a flat serve or kick serve? If you toss the tennis ball like you were trying to hit a flat serve, you could swing across the ball (a little bit) if you wanted to but, based on the way your arm, shoulder and body are built, you can’t swing across the ball as aggressively as if you had tossed the tennis ball further to your side. This toss location allows you to attack the tennis ball more aggressively and generate more side spin than you otherwise would be able to if you were tossing for a flat or kick serve.