John McEnroe says Donald Trump offered him $1,000,000 to play a tennis match against Serena or Venus Williams ... but he shot it down.
"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 .