Maria Sharapova, a five-time Grand Slam champion and the world’s highest-paid female athlete, announced Monday that she had tested positive for the recently banned drug meldonium at the Australian Open.

The tennis antidoping program confirmed the positive test, which occurred Jan. 26, the day Sharapova lost to Serena Williams in the quarterfinals. Sharapova, who has not played since because of a forearm injury, will be suspended provisionally Saturday pending a ruling in the case.

The commercial fallout was swift. Nike, one of Sharapova’s longtime sponsors, announced in a statement that it was suspending its relationship with her “while the investigation continues.” Sharapova has her own clothing line with Nike, with whom she signed an eight-year extension in 2010 that could reportedly be worth up to $70 million.

Sharapova, a 28-year-old Russian who is one of the world’s most visible sports figures, is by far the most prominent athlete to be barred for meldonium, a drug originally developed in Latvia for heart patients that aids blood flow and is not approved for sale in the United States.

But Sharapova is hardly alone. Athletes in several sports have reportedly tested positive for the substance since it was placed on the World Anti-Doping Agency’s banned list this year after being on its Monitoring List in 2015.

One of those who reportedly failed a test was Abeba Aregawi, a Swedish runner who won the women’s 1,500 meters at the 2013 world championships. Also on Monday, the Russian ice dancer Ekaterina Bobrova announced that she had tested positive for meldonium at the 2016 European Figure Skating Championships, where she and her partner, Dmitry Soloviev, placed third.

While expressing surprise, Bobrova told reporters that she was aware of meldonium being placed on the banned list, but Sharapova said she was unaware of the change before receiving notification of her positive test Wednesday.

“I take full responsibility for it,” she said Monday.

When Sharapova’s management group, IMG Tennis, sent word to the news media on Sunday about the news conference but supplied no details, speculation was widespread that she would help turn Monday into Retirement Day in sports, following Peyton Manning’s announcement in Denver.

She had already come back from shoulder surgery in 2008 and had played just three tour events and the Fed Cup final in the last eight months because of other injuries. But when she walked glumly to a lectern at a ballroom in a downtown Los Angeles hotel, a sheet of paper in hand, Sharapova promptly changed the expected story line by instead disclosing the failed test.

Sharapova said her family doctor began prescribing the drug mildronate, also known as meldonium, in 2006 after several health issues arose, including frequent cases of the flu.

“I was getting sick very often,” she said. “I had a deficiency in magnesium. I had irregular EKG results, and I had a family history of diabetes and there were signs of diabetes.”

Sharapova’s lawyer, John Haggerty, said in an interview after the news conference that the medication “brought these conditions that she had under control.”

Dr. Steven Nissen, chairman of the department of cardiovascular medicine at the Cleveland Clinic, said that the drug was not available in the United States but that it seemed to be used elsewhere to treat chest pains from severe heart disease.

“There is no way it would be clinically indicated in a healthy young athlete,” he said.

Sharapova indicated she continued to take the drug while regularly checking the list through the years to confirm that it was not prohibited. The World Anti-Doping Agency, in conjunction with the International Tennis Federation, sends an email to players at the end of each year with a link to a list of banned substances.

Sharapova confessed to not clicking on the link this time.

“I received a letter on Dec. 22 from WADA, an email with changes happening for next year as well as reporting your whereabouts and a link to a button where you can press to see the prohibited items for 2016,” she said. “I did not look at that list.”

Richard Ings, a former chief executive of the Australian Sports Anti-Doping Authority, said in an email Monday that “the smart story here is how Sharapova missed the memo of it being banned; she has a massive support team.”

“I think it’s very important to have a great team around you with coaches and doctors, but at the end of the day, everything you do is on you,” she said.

Russia has been one of the epicenters of the doping scandals that have rumbled through global sports of late. Its track and field athletes were suspended from international competition in November and may not be reinstated for the 2016 Summer Olympics.

Sharapova, though she continues to represent Russia, has been based in the United States since age 7, although she has, by her own admission Monday, been taking medication for a decade that is not approved in the United States.

“I’m pretty sure if this was a track and field athlete they’d be getting a much rawer deal than Sharapova,” Kelly Sotherton, the former British heptathlete and 400-meter runner, said in a Twitter message Monday.

Under the rules of tennis’s antidoping program, Sharapova’s positive test would not normally have been announced by the I.T.F. until the adjudication process had been completed. But she decided to make her case public and Haggerty said she did not intend to request that her B sample be tested.

“I thought it was very important for me to come out and speak about this in front of all of you, because throughout my long career, I have been very open and honest about many things,” she said. “I made a huge mistake, and I’ve let my fans down. I’ve let this sport down that I have been playing since the age of 4 and that I love so deeply.”

In higher doses, Haggerty said, meldonium can serve as a performance-enhancer. Because Sharapova was taking a prescribed dose for health purposes, he said he would probably request a minimal penalty from the I.T.F.

Ben Nichols, a WADA spokesman, said in an email Monday that meldonium had been moved from the monitored list to the banned list “because of evidence of its use by athletes with the intention of enhancing performance.”

One factor in its change in status was a 2015 study, funded in part by the Partnership for Clean Competition, that analyzed 8,300 urine samples collected at doping control sessions and found that 182 (2.2 percent) contained the substance.

But Haggerty said enhancing performance was never Sharapova’s intent.

“Maria was completely unaware there was any performance-enhancing capabilities to it whatsoever,” Haggerty said. “All the things, the medical conditions she was being treated for with this drug, had nothing to do with enhancing her performance. It had to do with her getting her health to a normal baseline.”

For a first offense, players can be barred for up to four years for intentionally ingesting a performance-enhancing substance. The maximum penalty for unintentionally using a prohibited drug is two years, Haggerty said, but he is holding out hope that no sanctions will be imposed.

“She acknowledged she took the drug called mildronate and that under a different name, meldonium, it is on the banned list,” Haggerty said. “And that is why she’s acknowledged that she’s failed the drug test, and now we are just going through the I.T.F. process to discuss with them why we believe that either no, or a very limited, sanction is required based upon all the facts surrounding why she was taking it, for how long she’s been taking it and the medical issues she was taking it for.”

Tennis’s antidoping program has sometimes been criticized for its failure to catch its stars. But the prominent men’s players Marin Cilic and Viktor Troicki were penalized in 2013, with both having their suspensions reduced on appeal. In the past two years, the program has significantly increased the number of blood tests and out-of-competition tests, although Sharapova’s positive test came in competition.

“She is a megastar,” Ings said. “So it ends the conspiracy theory that tennis has no stomach for big cases.”

Steve Simon, the chief executive of the Women’s Tennis Association, was complimentary but firm in a telephone interview after Sharapova’s announcement.

“Maria deserves a great deal of I think compliments because she showed a lot of grace and class and integrity today, which you expect from her by stepping up and acknowledging her mistake,” Simon said. “But I think it does show that the sport does have tremendous levels of integrity, and that there isn’t any athlete that is above the rules.”

But Sharapova’s suspension is clearly a major blow to the women’s game, and it remains unclear if and when Sharapova will play again.

“I don’t want to end my career this way,” she said, “and I really hope I will be given another chance to play this game.”

What Sharapova made clear Monday was that this was not the sort of setting in which she envisions ending it.

“I know many of you thought that I would be retiring today,” she said. “But if I was ever going to announce my retirement, it would probably not be in a downtown Los Angeles hotel with this fairly ugly carpet.”