Is an underhand serve legal in tennis?

Yes. The rules only require you to toss the ball by hand and hit it before it bounces, so an underarm serve is legal at every level.

What happens if a serve hits the net and goes in?

It is a let and you replay that serve. There is no limit on lets.

What happens if the serve hits the returner before bouncing?

The server wins the point, even if the returner was standing outside the court.

Tennis Serve Rules

7 video lessons

Where to stand, what is a fault, and what is a let.

Quick answer & tips ▾

You serve from behind the baseline, between the center mark and the sideline, starting from the right (deuce) side and alternating sides every point. Toss the ball by hand and hit it before it lands, into the service box diagonally across the net. You get two tries: miss both and it is a double fault, so you lose the point.

A serve is a fault if it lands outside the box, hits the net and stays on your side, if you step on the baseline before contact, or if you swing at the toss and miss. Catching a bad toss is not a fault. A serve that clips the net and lands in is a let and is replayed, as many times as it happens. Underarm serves are completely legal.

Key tips

Wait until the returner is ready before you serve.
Keep both feet behind the baseline until you hit the ball.
Catch a bad toss and start again; it costs nothing.
An underarm serve is legal and works against deep returners.

FAQ

Is an underhand serve legal in tennis?: Yes. The rules only require you to toss the ball by hand and hit it before it bounces, so an underarm serve is legal at every level.
What happens if a serve hits the net and goes in?: It is a let and you replay that serve. There is no limit on lets.
What happens if the serve hits the returner before bouncing?: The server wins the point, even if the returner was standing outside the court.