Here's a little history that shows why John McEnroe was right
to say Serena Williams would lose to a low-ranked male professional, and why Serena Williams so readily admitted it in 2013.
In 1998, a smoking, beer-drinking German
, 15 years older than Serena and her sister, Venus, beat both sisters like rented mules. And that was after a suds-soaked round of golf. Wikipedia has the details:
1998: Karsten Braasch vs. the Williams sistersAnother event dubbed a "Battle of the Sexes" took place during the 1998 Australian Open between Karsten Braasch and the Williams sisters. Venus and Serena Williams had claimed that they could beat any male player ranked outside the world's top 200, so Braasch, then ranked 203rd, challenged them both. Braasch was described by one journalist as "a man whose training regime centered around a pack of cigarettes and more than a couple bottles of ice cold lager". The matches took place on court number 12 in Melbourne Park, after Braasch had finished a round of golf and two beers. He first took on Serena and after leading 5–0, beat her 6–1. Venus then walked on court and again Braasch was victorious, this time winning 6–2. Braasch said afterwards, "500 and above, no chance". He added that he had played like someone ranked 600th in order to keep the game "fun". Braasch said the big difference was that men can chase down shots much easier, and that men put spin on the ball that the women can't handle. The Williams sisters adjusted their claim to beating men outside the top 350.
See Braasch’s own report here: How to... beat both Williams sisters in one afternoon,
by Karsten Braasch, Observer, September 2, 2001.
Now, why is it we so desperately want women in combat