Almost all of your joints are used in tennis, but the main ones in a tennis serve are the knee joints, and elbow joints.
You can hit an underhand serve in tennis; however, the overhand serve is more effective because it is more powerful. That is why you don't see the underhand serve used much.
a serve
Badly.
A tennis ball!
It depends on how hard you want to hit the ball
a tennis ball
Let
Yes
A tennis ball.
a fault
Fault