Maria Sharapova's multimillion dollar business empire has been plunged into turmoil as sponsors began severing ties with the Russian tennis star on Tuesday a day after she admitted failing a drug test.

The world's richest sportswoman announced on Monday that she had tested positive for Meldonium, a drug she said she had been taking since 2006 but was only added to the banned list this year.

Her sponsors were quick to react with US sportswear giant Nike, German luxury car maker Porsche and Swiss watchmaker TAG Heuer all halting their relationship with the former world number one.

The 28-year-old, whose rags-to-riches story was the stuff of Hollywood dreams, now risks a ban which could see her carefully constructed marketing strategy collapse.

The holder of five Grand Slams and 35 WTA titles, the Russian who arrived penniless with her family in the US, is as much a businesswoman as a sportswoman.

Despite winning just two WTA titles in 2015 she was the highest paid sportswoman that year, earning $30mn mostly from advertising, according to American magazine Forbes. Her fortune is estimated at $200mn.

In a sign of what the affair could mean to Sharapova's mass of money-spinning endorsements, Nike said it was "saddened and surprised" by the news.

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

Swiss watchmaker TAG Heuer followed by confirming they would not be renewing their marketing contract with Sharapova.

"Maria Sharapova was under contract with TAG Heuer until December 31th, 2015. We had been in talks to extend our collaboration," a company statement said.

"In view of the current situation, the Swiss watch brand has suspended negotiations and has decided not to renew the contract with Ms Sharapova."

Porsche became the third big-name sponsor to distance themselves, stating "until more details are known and we can analyse the situation, we have decided to suspend planned activities (with Sharapova)".

Sharapova signed a three-year deal to be brand ambassador for Porsche in April 2013. The Russian also won the WTA indoor tournament in Stuttgart, which is sponsored by the car manufacturers, in 2012, 2013 and 2014, with the champion driving off in a brand-new sports car.

'Huge mistake'  

Sharapova tested positive for Meldonium, a drug she said she had been taking since 2006 but was only added to the banned list this year. "I did fail the test and I take full responsibility for it," Sharapova said at a press conference at a downtown Los Angeles hotel on Monday.

"I made a huge mistake. I let my fans down. I let my sport down that I've been playing since the age of four that I love so deeply," added Sharapova, her voice wavering.

"I know that 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."

The ITF confirmed that Sharapova had tested positive on January 26 and had accepted the finding when she was notified on March 2.

"Ms Sharapova will be provisionally suspended with effect from March 12, pending determination of the case," the ITF said.

Her attorney, John Haggerty, said the positive test could carry a ban of up to four years, but that mitigating circumstances could see a lesser penalty.

Russian officials on Tuesday threw their backing behind Sharapova.

"I feel sorry for Masha. I hope that we will see her back on court and we are prepared to support her," sports minister Vitaly Mutko told state-run Tass news agency, using the Russian diminutive of Sharapova's first name.

"She travels from tournament to tournament and has a punishing schedule. The people in her team should be looking out for her."

Shamil Tarpishchev, head of the Russian tennis federation, said he believed Sharapova would still have a chance to play at the Rio Olympics in August.