Former world number one Maria Sharapova has revealed she failed a drugs test at the Australian Open.
The Russian, 28, tested positive for meldonium, a substance she has been taking since 2006 for health issues.
The International Tennis Federation (ITF) said the five-time Grand Slam champion would be provisionally suspended from 12 March.
Sportswear company Nike said it was halting its relationship with her until the investigation was complete.
"I did fail the test and take full responsibility for it," said Sharapova, who won Wimbledon as a 17-year-old in 2004.
Sharapova has been the highest-earning female athlete in the world in each of the past 11 years, according to the Forbes list.
With career earnings from tennis alone amounting to almost £26m, she claimed she had taken meldonium "for the past 10 years" after being given it by "my family doctor" but had known the drug as mildronate.
"A few days ago, after I received a letter from the ITF, I found out it also has another name of meldonium, which I did not know," she said.
Sharapova's lawyer, John Haggerty, told Sports Illustrated he was hopeful the player would avoid a lengthy ban.
"We think there is a laundry list of extremely mitigating circumstances that, once taken into consideration, would result in dramatically reducing any sanction that they might want to impose on Maria," he said.
How did this happen?
Sharapova provided the anti-doping sample in question on 26 January, the day she lost to world number one Serena Williams in the quarter-finals of the Australian Open in Melbourne.
The World Anti-Doping Agency (Wada) analysed the sample and returned a positive for meldonium.
Sharapova, who lives in Florida, was subsequently charged on 2 March.
"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 that medicine for the past 10 years," she said.
"But, on 1 January, the rules changed and meldonium became a prohibited substance, which I had not known."
She added that Wada had sent her an email on 22 December informing her of changes to the banned list, but she had failed to "click" on the link that would have detailed the prohibited items.
Was this news a surprise?
Completely. There was speculation she had called Monday's news conference, which was streamed live online, to announce she was quitting tennis.
"I know many of you thought that I would be retiring today, but if I was ever going to announce my retirement it would not be in a downtown Los Angeles hotel with this fairly ugly carpet," she said.
According to BBC tennis commentator Andrew Castle, Sharapova's positive drugs test is "a hammer blow to the sport".
Three-time Grand Slam singles champion Jennifer Capriati said she was "extremely angry and disappointed".
The American, whose career was ended by injury, added: "If this medication helped me to come back again, would everyone be all right with me taking it?
"In my opinion, if it's all true every title should be stripped. This is other people's lives as well."
However, 18-time Grand Slam champion Martina Navratilova felt Sharapova had simply committed an "honest mistake".
What is meldonium?
It is meant for angina patients but athletes like it because it helps their endurance and ability to recover from big efforts.
It is on the banned list now because Wada started seeing it in lots of samples and found it does have performance-enhancing properties.
It was on Wada's 'watchlist' for over a year and added to the banned list on 1 January.
Made in Latvia, it is widely available - without prescription and at low cost - in many east European countries, but it is not licensed in most western countries, including the United States.
It is thought that hundreds of athletes have been using it and there are a lot more cases in the pipeline.
Why did Sharapova take it?
Haggerty said she started to take meldonium after her doctor did "an extensive battery of tests to determine what medical conditions were causing her to be sick on a frequent basis".
She had "abnormal electrocardiogram readings" and "some diabetes indicators", which prompted the doctor to recommend medication, including meldonium.
He added: "She took it on a regular basis as recommended by her doctor. He told her what to take and when to take it, then continued to test her and confirm that it was giving her the desired improved medical condition."
Wada placed meldonium on its monitoring programme in 2015 before adding it to the banned list this year "because of evidence of its use by athletes with the intention of enhancing performance".
What happens now?
Sharapova could apply for a retroactive therapeutic use exemption (TUE).
A TUE allows a player to use a banned substance, without committing an anti-doping rule violation, if they have a medical condition that requires it.
As for her sponsors, it remains to be seen if others follow the lead of Nike, who said it was "saddened and surprised by the news".
She also has contracts with Evian, Tag Heuer, Porsche and Avon.
Whatever happens, Sharapova, who turns 29 in April, says she hopes to be able to return to tennis in the future.
"I made a huge mistake," she said. "I have let my fans down and let the sport down that I have been playing since the age of four, that I love so deeply.
"I know that with this I face consequences and I don't want to end my career this way. I really hope to be given another chance to play this game."
Sharapova first reached world number one in August 2005 and is currently seventh in the rankings.
But she has played just four tournaments since Wimbledon last July as she struggled with an arm injury.
How long could a ban be?
But Jeff Tarango, Sharapova's former coach and an ex-Tour professional, said he doubted she would be banned for that long.
"I think it immediately falls under two years, but, with these circumstances, probably one year," the American told BBC Radio 5 live.
"She can apply for a TUE. If it really is something she had to take for her heart and diabetes ,then it falls under a TUE."
Several high-profile Tour players have been suspended for anti-doping violations, among them Marin Cilic, Viktor Troicki and Barbora Strycova.
Former US Open champion Cilic was banned for taking a glucose tablet bought at a pharmacy in France but had his suspension reduced from nine to four months in October 2013.
Troicki was suspended for 12 months on appeal after refusing to take a blood test at a tournament in Monte Carlo in 2013, claiming he was feeling unwell and had a phobia of needles.
Czech player Strycova was given a back-dated six-month ban in 2013 after saying a banned stimulant entered her system via a weight-loss supplement.
Former Grand Slam winners Martina Hingis and Andre Agassi are among those who have been banned for testing positive for recreational drugs.
What's the WTA's stance?
Women's Tennis Association (WTA) president Steve Simon said he was "very saddened" at Sharapova's failed test.
"Maria is a leader and I have always known her to be a woman of great integrity," he added.
"As Maria acknowledged, it is every player's responsibility to know what they put in their body and to know if it is permissible.
"This matter is now in the hands of the Tennis Anti-Doping Programme and its standard procedures.
"The WTA will support the decisions reached through this process."
Tennis Australia added it was "surprised" by the news.
It added: "Throughout her career Maria has always impressed with her professionalism as a leader and role-model in our sport."