Russian tennis star Maria Sharapova said on Monday she had failed a drug test at the Australian Open due to a substance she was taking for health issues, leading longtime sponsor Nike to announce it was suspending ties during the investigation.

The 28-year-old Sharapova, a five-time grand slam champion and the highest paid woman in sports, will be provisionally suspended starting March 12, the International Tennis Federation (ITF) said.

Nike, the world's largest sportswear maker, said it was "saddened and surprised" by the news and released a statement saying it was putting ties on hold with the player.

"We have decided to suspend our relationship with Maria while the investigation continues," it said.

Watchmakers TAG Heuer also pulled their support for the tennis star.

"Maria Sharapova was under contract with TAG Heuer until December 31, 2015," a statement read. "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 with Ms Sharapova."

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.

"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."

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.

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