If you touch the net while playing a shot, you lose a point. (I remember seeing Federer lose a point a few years ago, as he fell on the net after hitting a smash shot.) However, a player could stand close to the net without touching it and stretch his playing arm into the opponent's court to play a shot. This seems to offer an unfair advantage, because the net is taken out of the picture, and the player only needs to aim the ball correctly into the opponent's court. Is this allowed?
A player is only allowed to stretch into the opponent's court in 2 situations:
The player is near the net and they hit the ball after it has passed over the net. The follow through on their swing is allowed to extend into the opponent's court (provided that they do not touch the net or the ground).
The ball bounces on the player's side of the court, and due to spin or wind, it goes back over the net. In this situation, the player is allowed to reach over the net and hit the ball (provided that they do not touch the net or the ground).
Here are a couple sources:
A player may break the plane of the net on a follow through from a shot as long as the ball was on that player’s side of the court when the ball was struck. (The player can only reach over to play a ball in the situation stated in the paragraph below).
If the spin or wind brings the ball back over the net to the side of the player(s) who hit the shot, the opponent(s) may then reach over the net and play the ball. They may not touch the net or the opponent’s court. This is the only situation when a player may reach over the net to play a ball. If the opponent does not play the ball and it bounces on the side of the player who hit the spin shot, that player who hit that shot wins the point.
The player in either situation may not touch the net, or the opponent’s court with anything he wears or carries or with any part of the body.
The point is lost if:
g. The player or the racket, whether in the player’s hand or not, or anything which the player is wearing or carrying touches the net, net posts/singles sticks, cord or metal cable, strap or band, or the opponent’s court at any time while the ball is in play; or
h. The player hits the ball before it has passed the net;