The Ultimate Head Game

If 90 percent of baseball is half-mental, as Yogi Berra famously quipped, what percentage of tennis is? In a sport in which the (arguably, if you insist) greatest player to ever play the game can double-fault away his ticket to the US Open final, or where entire careers at the highest level are dictated by mental health, there is much more happening in the cerebral cortex than there is on the court.