Rules

22. Ball Falls on Line
A ball falling on a line is regarded as falling in the Court bounded by that line.

USTA Comment: In a non-officiated match, each player makes the call on any ball hit toward his side of the net. If a player cannot call a ball out with certainty he should regard it as good. In doubles, normally the Receiver's partner makes the calls with respect to the service line, with the Receiver calling the side and center lines, but either partner may make the call on any ball he clearly sees out.