Serena Williams is the best of her gender, but the 23-time Grand Slam winner will be way behind if put beside a man.

Speaking as part of a tour to promote his new book "But Seriously", McEnroe said he would place Williams "like 700 in the world" on the men's circuit.

"Best female player ever -- no question.

"If she had to just play the circuit -- the men's circuit -- that would be an entirely different story," McEnroe told American broadcaster National Public Radio.

The seven-time major winner said the best thing Williams would have going for her on the men's tour would be her mental toughness.

"The reality of what would happen on a given day is Serena could beat some players, I believe, because she is so incredibly strong mentally."

He said her rivals would crack under pressure.

"She could overcome some situations where players would choke because she's been in it so many times, so many situations at Wimbledon, The US Open, etc," he said.

Williams became the oldest woman to win a Grand Slam title when she claimed the 2015 Wimbledon title at age 33. The Los Angeles native has 23 Grand Slam singles crowns and 14 doubles titles in majors.