Juggler's Tennis is the second trick most jugglers
attempt after having learned the Three Ball Cascade. It gets its name from
the one ball that travels back and forth over the others, resembling a
tennis match. Overall, Juggler's Tennis is an easy trick, only marginally
harder than the Cascade.
To start learning any pattern, it is always best to break the trick down into its core features. In this case, the only difference between Juggler's Tennis and the Cascade is one ball being constantly overthrown, a term used to describe a ball that travels over the others, as opposed to beneath them.
Thus, the best way to learn Juggler's Tennis is to practice doing overthrows. Starting with a standard Cascade, you are going to want to make one of your throws higher than the rest, allowing that ball to travel above the previous ball instead of underneath it. It is probably best to start learning overthrows with your dominant hand, but eventually you will need to be proficient with both hands. A good way to practice would be to make one overthrow, wait maybe three or four throws, and then make another overthrow, this time with the opposite hand:
Once you have the occasional overthrow down, you can begin to decrease the number of normal "recovery throws" that you have in between your overthrows. In order to be doing Juggler's Tennis, the same ball must be constantly overthrown—at no time should any other ball be thrown over it.