John McEnroe isn't as funny as he thinks he is.

Ahead of the U.S. Open (of which he won four titles throughout his career), the tennis legend appeared on Jimmy Kimmel Live! Wednesday and was asked about Serena Williams, who will try for calendar grand slam when the tournament begins later this month. Winning the Australian Open, the French Open and Wimbledon was no easy feat, and winning the U.S. Open will not be a walk in the park, either. "The last player to do that was Steffi Graf, who's probably the second greatest player that ever lived, to me, behind Serena," McEnroe said. "She did that in 1988. Before that it was done, I think, in 1950. So, this is very, very rare."

McEnroe told Jimmy Kimmel he has "hit with Serena" before. Kimmel asked, "If you guys played a serious match, who do you think would win that match?"

"My daughters are over there. I think it's mixed. I think they'd probably think I'd lose to Serena," the former World No. 1 admitted to the late-night host. Given the chance to play against Williams, McEnroe said, "I believe that I could still take her."

They nearly faced each other once before.

"Fifteen years ago, Donald Trump—who seems to be in the news a little bit—he made an offer, which I felt like was not enough. I think he was humoring the two of us. Serena, she's got a lot to lose by losing to an old fart like myself," he said. Then, making a sexist joke, McEnroe added, "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."

McEnroe said he would consider playing tennis with Williams for big bucks, similar to Manny Pacquiao and Floyd Mayweather's televised fight. They would have to stick to their own sport, though. "I don't think I could take her in the ring," he said.

Jokes aside, McEnroe wasn't sure he'd be able to keep up with today's tennis players. "No singles players really play doubles anymore. I thought that doubles was a good way for me to sort of practice and get some reps in. I didn't like to train in the gym as much as some of the players these days," the sportsman, 56, said.

"Nowadays they've figured a way to max out. They're like machines now," McEnroe continued. In his heyday, he said, "I wanted to be a little bit of like a rock star tennis player. Like, could you possibly be No. 1 and to some degree burn the candle at both ends? The answer was yes for a while, but it caught up to me eventually."

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