Naomi Osaka is being sued by her former coach who says the world No. 1 tennis player owes him 20 percent of her career earnings, American entertainment website TMZ has reported.

Christophe Jean, the two-time Grand Slam champion's former junior coach is demanding a least $2 million of the $10.8 million prize money Osaka has collected during her career, TMZ reported on Monday. He claims he was hired by Osaka's father, Leonard Francois, who he says signed a contract promising him the money.

According to the lawsuit he filed in Florida's state court, Jean coached Osaka and her sister, Mari, back in 2011. He said Francois was unable to pay him at the time, so they agreed to a contract, which granted Jean 20 percent of their prize money and endorsements for an "indefinite" period.

Osaka's lawyer, Alex Spiro, was quoted by TMZ as calling the so-called contract "absurd" as Osaka has never seen it, but added that he understands that "Naomi's meteoric rise as an international icon and inspiration would lead to some false claim."

The 21-year-old sensation became the 10th woman in history to win the U.S. Open and Australian Open back-to-back, and the first-ever Asian world No. 1 following her triumph at January's Australian Open.

Earlier this month, Osaka missed her bid in her first title defense after crashing out in the fourth round of the BNP Paribas Open in Indian Wells,