What counts as a foot fault, and who calls it?

Under Rule 18 of the 2026 ITF Rules of Tennis, during the service motion the server must not:

1. Change position by walking or running (slight movements of the feet are allowed);
2. Touch the baseline or the court with either foot;
3. Touch the area outside the imaginary extension of the sideline with either foot; or
4. Touch the imaginary extension of the centre mark with either foot.

• In singles, the server may not stand behind the part of the baseline between the singles and doubles sidelines. In doubles this is allowed.
• The server may have one or both feet off the ground.
• A foot fault is a service fault.

Who calls it: where line umpires are assigned, the line umpire on that line calls foot faults. Where there is no line umpire for that line, the chair umpire calls it.