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

Williams and Sharapova were not always on great terms, each admitted in their speeches, but came to share a mutual respect and a growing friendship since retiring.

Maria Sharapova, a five-time Grand Slam champion, was inducted into the International Tennis Hall of Fame earlier this year, alongside legendary American doubles champions Bob and Mike Bryan.