At 56, John McEnroe believes he can beat Serena Williams

John McEnroe cannot be serious.

Appearing on "Jimmy Kimmel Live" Wednesday, the 56-year-old said he thinks he could beat Serena Williams today if the two tennis greats played a serious match against each other.

"My daughters ... They think I'd probably lose to Serena," said McEnroe, who retired from the men's pro tennis tour in 1992. "I believe that I could still take her."

McEnroe — who ranked Williams the greatest women's player of all time and Steffi Graf No. 2 — then said that, 15 years ago, Donald Trump approached him and Williams about a match.

"He made an offer, which I felt like was not enough. I think he was humoring the two of us," the seven-time Grand Slam champion said. "And, Serena, she's got a lot to lose by losing to an old fart like myself. And I've got a lot to lose because if I lose to, God forbid, a woman, then I'm not allowed in any men's locker room for the next 15 years or possibly the end of my life. That's part of why it's intriguing."

After McEnroe said he'd play Williams for the kind of money that Floyd Mayweather Jr. and Manny Pacquiao did for their fight, Kimmel joked they could square off as part of a doubleheader with Mayweather-Pacquiao II.

"I don't think I could take her in the ring," McEnroe said.

He appeared to be serious.

Check out McEnroe's appearance on Kimmel below.