Naomi Osaka almost topped the Forbes' highest-earning female athlete list, slotting in second behind fellow tennis star Serena Williams in the annual ranking of the top 10 paid sportswomen.

According to the ranking released Tuesday, Osaka's total earnings were $24.3 million for the 12-month period ending June 1, including an estimated $16 million off-court income, while Williams secured the top spot for the fourth year in a row with $29.2 million.

The 21-year-old Osaka, who has a Japanese mother and a Haitian father, has endorsement deals with Mastercard Inc., All Nippon Airways Co., Nissan Motor Co., among others.

She won two majors, the U.S. Open and the Australian Open, and became the first Asian tennis player to reach world No. 1 in singles.

"Osaka's accomplishments, youth, skill and multicultural appeal make her a marketer's dream," Forbes said.

German tennis player Angelique Kerber was the third highest-paid female athlete in the world, with $11.8 million.

The 10 highest-paid female athletes in the world in 2019 are all tennis players.