Russian tennis star Maria Sharapova, the highest-paid woman in sports, said on Monday that she failed a drug test at the Australian Open due to a substance she has been taking for 10 years for health issues.
The 28-year-old Sharapova, a five-time Grand Slam champion, will be provisionally suspended starting March 12, the International Tennis Federation (ITF) said.
One of her biggest sponsors, Nike Inc., said it was suspending ties while the case is being investigated.
She is the seventh athlete in a month to test positive for meldonium, which is used to treat diabetes and low magnesium, and was only banned by the World Anti-Doping Agency (WADA) as of January 1.
In its statement to put ties on hold that came just hours after the tennis star's announcement, Nike, the world's largest sportswear maker, said, "We are saddened and surprised by the news about Maria Sharapova."
Swiss watch brand TAG Heuer is dropping its sponsorship of Sharapova, it said on Tuesday after the Russian tennis star said she had failed a drug test.
"Maria Sharapova was under contract with TAG Heuer until December 31, 2015. We had been in talks to extend our collaboration. In view of the current situation, the Swiss watch brand has suspended negotiations and has decided not to renew the contract," the unit of French luxury goods group LVMH said in a statement.
The ITF's anti-doping program calls for a four-year suspension for a positive test, but that ban can be reduced in various circumstances, such as for first-time offences or if the player shows no significant fault or negligence.
If a player bears no fault or negligence, there is no suspension.
"I made a huge mistake. I let my fans down and I let the sport down," said Sharapova, a teenage tennis prodigy who became the third-youngest Wimbledon champion. "I take full responsibility for it."
"I know that with this I face consequences and I don't want to end my career this way. I really hope that I will be given another chance to play this game," former world number one Sharapova told a news conference in a downtown Los Angeles hotel.
According to Forbes, she earned $29.5 million in 2015, mostly from endorsements.
Sharapova said her family doctor had been giving her mildronate, which is also called meldonium, for 10 years after she frequently became sick, had irregular EKG results, a magnesium deficiency and a family history of diabetes.
"It is very important for you to understand that for 10 years this medicine was not on WADA's banned list and I had been legally taking the medicine. But on January the first, the rules have changed and meldonium became a prohibited substance."
WADA declined to comment until ITF issues a final decision.
Steve Simon, CEO of the Women's Tennis Association, said in a statement he was saddened to hear the news.
"Maria (Sharapova) is a leader and I have always known her to be a woman of great integrity," he said.
"Nevertheless, as Maria acknowledged, it is every player's responsibility to know what they put in their body and to know if it is permissible. The WTA will support the decisions reached through this process."