The business publication calculated that the 23-time Grand Slam singles champion earned $29.2 million in the 12-month period ending June 1 -- with just $4.2 million of that coming from prize money.
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Japan's Naomi Osaka, who burst to international stardom with a stunning upset of Williams in the 2018 US Open final and went on to win the Australian Open in January, became just the fourth woman to earn more than $20 million in a year -- after Williams and tennis stars Maria Sharapova and Li Na.
Forbes pegged Osaka's earnings at $24.3 million, putting her second on a list dominated by tennis players.
Former Wimbledon champion Angelique Kerber was third on the list with earnings of $11.8 million followed by this year's Wimbledon champion Simona Halep.
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Forbes included prize money, salaries, bonuses, endorsements and appearance fees between June 1, 2018, and June 1, 2019 in the figures.
The highest placed athlete from a different sport was US footballer Alex Morgan, who clocked in at 12th. Morgan earned $5.8 million, most of it in endorsements.
Indian badminton star PV Sindhu and Thai golfer Ariya Jutanugarn were the other non-tennis players in the top 15.
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