From section 18 of the USTA rule nook
18. FOOT FAULT
During the service motion, the server shall not:
a. Change position by walking or running, although slightmovements of the feet
are permitted; or
b. Touch the baseline or the court with either foot; or
c. Touch the area outside the imaginary extension of the sideline with either
d. Touch the imaginary extension of the centre mark with either foot.
If the server breaks this rule it is a “Foot Fault”.
"You should be playing linebacker, not singles."