Maria Sharapova
Russian tennis champion Maria Sharapova stands to lose out on her multi-million endorsement contracts following her admission that she was consuming a drug for 10 years that was added to the list of banned drugs by WADA in January this year. Picture: Maria Sharapova at the press conference where she revealed she tested positiveReuters

Russian tennis star Maria Sharapova, the world's highest-earning female athlete in 2015 for the 11th straight year, is no longer the brand ambassador of Swiss watch-maker Tag Heuer and American sportswear brand Nike following she testing positive for a banned substance she had been taking for 10 years.

While Tag Heuer said it would not renew its contract with the star, Nike said it was "saddened and surprised" with the 28-year-old's admission (to consuming the banned substance), reports the BBC.

"We have decided to suspend our relationship with Maria while the investigation continues," the news channel quoted Nike as saying.

Earlier, the five-time Grand Slam champion was provisionally banned from Australian tennis after she disclosed Monday about taking the banned drug, reports The Guardian.

Sharapova was consuming mildronate, a drug that also goes by the name meldonium. She said she was unaware of the fact that it was added to the list of banned drugs by the World Anti-Doping Agency (WADA) in January this year.

She claimed she was taking the drug since it was prescribed by her doctor about 10 years ago to deal with her irregular heartbeat and a history of diabetes in her family, the daily said.

Her earnings were about $30 million in 2015 from endorsements and winnings. Her association with Nike started when she was just 11 years of age.

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Sharapova said in Los Angeles at a press conference Monday she took full responsibility for the development though the substance was banned only in January this year.

"It is very important for you to understand that for 10 years this medicine was not on Wada's banned list and I had legally been taking the medicine for the past 10 years. But on Jan. 1 the rules had changed and meldonium became a prohibited substance which I had not known. I failed the test and I take full responsibility for it. I made a huge mistake," the Guardian quoted her as saying.

"I let my fans down, I let the sport down that I have been playing since the age of four and I love so deeply. I know with this I face consequences and I don't want to end my career this way and I really hope I will be given another chance to play this game."

Ironically, the news came on the eve of International Women's Day.