Retired tennis player John McEnroe said Serena Williams would be “like 700” in the world tennis rankings if she played in the men's tour.

Mr McEnroe, speaking to NPR about his memoir But Seriously, said Ms Williams was the best female player ever, “no question.”

But when asked if he thought she was the best ever tennis player, without a gender qualifier, Mr McEnroe did not agree.

He said: “If she played the men's circuit she'd be like 700 in the world.”

The former tennis player added that he thought the 35-year-old could win against certain male counterparts “but if she had to just play the circuit - the men's circuit - that would be an entirely different story.”

Mr McEnroe, the bad boy of the tennis court who famously yelled “You cannot be serious” when the umpire said the ball was out, won seven Grand Slam titles in his career.

Ms Williams has won 23.

Serena Williams challenges two strangers to a match

She also won the Australian Open final in January this year, defeating her sister Venus Williams, while eight weeks pregnant.

Mr McEnroe added: “Maybe at some point a women's tennis player can be better than anybody. I just haven't seen it in any other sport, and I haven't seen it in tennis. I suppose anything's possible at some stage.”

He insisted he had four daughters and he was a feminist.

When asked if he believed he could beat Ms Williams in a match when he was nearing 60 years old, he said he would “train very seriously”.

“Absolutely – to try and be as ready as I possibly could, but I bring things to the table, certainly until recently,” he said.