Novak Djokovic thinks he came up with a way to get Serena Williams back on the tennis tour: Dare her to do it.

Serena Williams and Maria Sharapova were rivals during their tennis careers, but they are now friends in retirement.

Though Williams and Sharapova were rivals both on and off the court throughout their storied careers, they have unexpectedly emerged as friends since retiring. With their relationship now a friendship, Williams not only surprised Sharapova by attending her Hall of Fame induction, but gave a touching speech in her honor.