Trump Offered Me $1 Mil

... to Play Against Serena

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 ... 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…