Naomi Osaka is reportedly being sued by her former coach Christophe Jean, who worked with the two-time Grand Slam winner when she was 13 and wants 20 percent of her earnings.

According to TMZ, Jean says he signed a contract with Osaka's father, Francois, who he says could not afford to pay the coach for his services training both Osaka and her sister, Mari.

Jean says the contract entitles him to 20 percent of Naomi and Mari Osaka's "tennis prize money and endorsement deals forever." He believes he's due $2 million (£1.5 million), per TMZ.

“While it comes as no surprise that Naomi's meteoric rise as an international icon and inspiration would lead to some false claim, this silly, imaginary contract that Naomi never saw or signed—which purports to give away part of herself at the age of 14—is particularly absurd," Alex Spiro, Osaka's lawyer, said in a statement, via Dave Hyde of the South Florida Sun Sentinel.

Osaka has enjoyed a remarkable rise that has taken her to the top of the women's tennis rankings.

She won her first Grand Slam at the U.S. Open in September 2018, shocking six-time champion Serena Williams in straight sets in the final.

The victory also saw Osaka etch her name into the history books:

The 21-year-old then followed that up by winning the Australian Open in January. Her three-set victory over Petra Kvitova also saw her become the new world No. 1:

Osaka's success has brought her more than $10.8 million (£8.1 million) in prize money to date, according to the WTA.

She is being tipped to add plenty more Grand Slam titles to her collection:

Osaka has had coaching issues this season. She split with Sascha Bajin in February but said the decision was not for financial reasons, per BBC Sport.

"Everyone thinks it was a money-related issue, but it wasn't," she said. "I wouldn't put success over my happiness. I'm not going to sacrifice that just to keep a person around. That's one of the most hurtful things I've ever heard."

Osaka hired Jermaine Jenkins, a former hitting coach of Venus Williams, as her new coach in March, per the Associated Press (h/t ESPN).