John McEnroe says while he has great respect for Serena Williams as the best womens player, the 23-time Grand Slam winner would be the 700th ranked player in the men's game.
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," McEnroe told American broadcaster National Public Radio.
"If she had to just play the circuit - the men's circuit - that would be an entirely different story."
When asked to qualify his comments McEnroe explained where he thought Williams would stand on the men's circuit.
"Well because if she was in, if she played the men's circuit she'd be like 700 in the world," McEnroe said.
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," added McEnroe, a three-time Wimbledon champion.
McEnroe 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 33.
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