Serbian Tennis phenom Novak Djokovic used to be more famous for clowning around than for winning tournaments. On YouTube, you'll find Djokovic (pronounced Joke-oh-vich) performing a karaoke "I Will Survive." He once modeled undies at a fashion show. His impressions of Rafael Nadal, John McEnroe and Maria Sharapova are crowd favorites. "I like to laugh my ass out," Djokovic says. (When you juggle four languages, as he does, you're allowed to jumble a phrase or two.)
But he's not just the Djoker anymore. At a routine hitting session in Los Angeles one August afternoon, he's firing shots that have his sparring...