Coach Kimbo Tennis
Immediately before starting the service motion, the server shall stand at
rest with both feet behind (i.e. further from the net than) the baseline and
within the imaginary extensions of the centre mark and the sideline.
The server shall then release the ball by hand in any direction and hit the
ball with the racket before the ball hits the ground. The service motion is completed at the moment that the player's racket hits or misses the ball. A player who is able to use only one arm may use the racket for the release of the ball.
USTA Comment 16.1: May a player serve underhanded? Yes.
There is no restriction in the rules on the kind of service motion that a
server may use.