Even so, Forbes reported that Williams earned an estimated $18 million past year because of deals with the likes of Nike, JPMorgan Chase, Gatorade and others.
Mayweather, Ronaldo and Tiger Woods are the only three athletes to have topped the list over the past 18 years, Forbes said.
Forbes used salary and bonus data between June 1, 2017 and June 1, 2018 to compile the list.
Tennis star Serena Williams, who previous year was the only woman to feature on the list at 51st place with earnings of $36 million, dropped out of the rankings after taking a break from the sport due to the birth of her daughter. After the windfall from boxing Floyd Mayweather, McGregor's promotional stock has surely risen with the UFC and he could have easily surpassed $13 million in salary and endorsements no matter whom he might have fought in Brazil.
American business magazine Forbes released its annual report of the highest paid athletes in sport and not a single woman made the list, a first since 2010. The 29-year-old batting sensation earned $24 million over the last 12 months; $4 million through salary and winnings, and $20 million through brand endorsements.
"Boxing's biggest night of 2017 helped Mayweather and McGregor earn almost $400 million combined", Kurt Badenhausen, senior editor, Forbes Media said in a statement on Tuesday. Forbes said tennis' female stars Li Na, Maria Sharapova and Serena Williams were list regulars but Li retired in 2014 and Sharapova is still dealing with the aftermath of a 15-month suspension for using a banned substance.
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Completing the top 10 were basketball's LeBron James ($85.5M), tennis' Roger Federer ($77.2M), basketball's Stephen Curry ($76.9M) and quarterbacks Matt Ryan ($67.3M) and Matthew Stafford ($59.5M).
NBA stars lead the way with 40 basketball players among the top 100.
Atlanta Falcons quarterback Matt Ryan is the highest earning National Football League player, coming in at ninth with $88 million.
There are no female athletes on the list.
The lowest paid athlete on the list was basketball star Nicolas Batum with $22.9 million earned in the time frame.