Tennis superstar Serena Williams is not to be messed with, on or off the court. Over the weekend, John McEnroe, while promoting his new memoir, But Seriously, told NPR he believes Williams is the best female player ever. When asked why he didn't refer to her, like others have, as the best player in the world, McEnroe responded that while she is "incredible," if Williams "played the men's circuit, she'd be like 700 in the world." Williams waited until Monday to tweet a message right to McEnroe.