Rules

17. The Ball in Play
A ball is in play from the moment at which it is delivered in service. Unless a fault or a let is called it remains in play until the point is decided.

USTA Comment: A point is not decided simply when, or because, a good shot has clearly passed a player, or when an apparently bad shot passes over a baseline or sideline. An outgoing ball is still definitely in play until it actually strikes the ground, backstop, or a permanent fixture (other than the net, posts, singles sticks, cord or metal cable, strap or band), or a player. The same applies to a good ball, bounding after it has landed in the proper Court. A ball that becomes imbedded in the net is out of play.

USTA Comment: When a ball is hit into the net and the player on the other side, thinking the ball is coming over, strikes at it and hits the net he loses the point if his touching the net occurs while the ball is still in play.

Case 1. A player fails to make a good return. No call is made and the ball remains in play. May his opponent later claim the point after the rally has ended?
Decision: No. The point may not be claimed if the players continue to play after the error has been made, provided the opponent was not hindered.

USTA Comment: An out call on A's shot to B's Court must be made before B's return has either gone out of play or been hit by A. See Case 3 under Rule 29 regarding this situation in an umpired match.