Excerpts from the USTA Rules of Tennis:
Rule 16. THE SERVICE
Immediately before starting the service motion, the server shall stand at
rest with both feet behind (i.e. further from the net than) the baseline and
within the imaginary extensions of the centre mark and the sideline
... The service motion is completed at the moment that the player’s racket hits or misses the ball.
Rule 18. FOOT FAULT
During the service motion, the server shall not:
a. Change position by walking or running, although slight movements 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 foot; or
d. Touch the imaginary extension of the centre mark with either foot.
If the server breaks this rule it is a “Foot Fault”.
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If you read the rules carefully, you will see that you are not permitted to touch the center mark (or an imaginary extension of that mark) until the ball is contacted (or missed). However, if you are up in the air and cross that center line before contact, I believe that a foot fault has not occurred.
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