Hall of Famer Billie Jean King won an astounding 39 Grand Slam titles in singles, doubles and mixed doubles.

The tennis legend has played and seen a lot of champions in her time. So King's opinion carries authority as she asserts Maria Sharapova and Milos Raonic are two of the hardest-working athletes in sport.

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In her column, The Value of Work Ethic, published on The Players Tribune.com, King writes she knew Sharapova would rise to world No. 1 from the moment they first met.

"From the first time I saw [Maria] I knew she was destined to become the No. 1 player in the world because she does have a real gift of talent packaged with a drive that is second to none," King writes.

The former world No. 1 points to Sharapova's improved footwork as a primary reason she transformed herself into a French Open champion after some early-career struggles on clay.

"Specifically, Maria has focused on improvements to her footwork, committing to improving her outside leg on her backhand side," King writes. "I think this change alone helped her win the French Open — a major championship on what used to be her most challenging surface."

Former world No. 3 Ivan Ljubicic, who is Raonic's co-coach along with Riccardo Piatti, tells King the sixth-ranked Canadian is a "workaholic", whose work ethic is unmatched on the ATP Tour.

“Milos is by far the most determined and workaholic person I have ever met,” Ljubicic told King. “We literally have a problem getting him to rest and that is something we are always working on. His determination is so intense and the level of focus he puts in practice sessions is unmatched in tennis today, at least from what I see. So it is a challenge for us to manage this and allow him to use it to reach his goals.”