Williams' surprise appearance to induct Sharapova was "months in the making" and required the code name of another female icon.

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.

Maria Sharapova has entered the International Tennis Hall of Fame, with Serena Williams introducing her at the ceremony on Saturday night.