Tennis legend John McEnroe says that while he has great respect for Serena Williams as the best player of her gender, the 23-time Grand Slam winner would be the 700th-ranked player on the men's tour.

Speaking as part of a tour to promote his new book But Seriously, McEnroe said he would place Williams "like No 700 in the world" on the men's circuit.

"Best female player ever - no question. But if she had to just play the circuit - the men's circuit - that would be an entirely different story," McEnroe said in New York on the weekend.

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 some 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.

The ATP Tour rankings only go to the top 500 and Andy Murray is the current top-ranked ATP Tour player while Temur Ismailov of Uzbekistan holds down the 500th spot.

Agence France-Presse