But here’s a fun thing: I got my hands on McEnroe’s first professional tournament, the 1978 Forest Hills clay court event.
That was not his first professional event, McEnroe was already ranked in the top 20 at the time of that match, July '78.
He would finish that year #4. His first pro event was in '76.
He seems to have made a big change in his service motion sometime from July '78 & Jan '79(when he won the Masters)