"I was calling a match and suddenly I get this envelope and it's from Donald Trump," McEnroe said on "In Depth with Graham Bensinger."
McEnroe didn't specify WHEN Trump sent the envelope -- only noting it was before he became President.
So, why'd John turn it down?
"It's never been something where I was like ... I wanna go play a woman in a match."
You gotta watch the clip ... John is pretty confident he'd win -- reminding everyone he was once the #1 player in the world.
Still, his own kids aren't convinced he'd beat Serena, which begs one question -- IS IT TOO LATE TO CHANGE YOUR MIND?!?!