Last year, I interviewed Maria Sharapova at a party in Miami that was held to celebrate her second year as a global brand ambassador for Porsche. It was one of the stranger interviews I have ever conducted: it was done standing up, on the red carpet that led her past the paparazzi and into the party, which was thick with bronzed Eurotrash. I had been offered the interview by Sharapova’s P.R. team and was given 10 minutes to speak with her. The peculiar setting threw me a bit, as did her height. I am over six feet tall, and I was dwarfed by her (true, she was in very high heels, but still . . .).

But it turned out to be a terrific interview. She was smart, funny, and totally engaging. Candid, too: she told me that she had no desire to become a commentator or coach after retiring. “I love the sport, but I don’t see myself going into the commentary booth,” she told me. “Except for when my boyfriend”—Grigor Dimitrov—“plays, I don’t remember when I actually sat and watched a whole match on television. I have a short attention span for that.” Instead, she was intent on building a business empire. She said that she loved learning the ins and outs of the brands that she was involved with, such as Porsche, and that her future would revolve around entrepreneurship. With her huge endorsement portfolio and her Sugarpova brand, which began as a line of candies and branched out to clothing and fashion accessories, Sharapova was a budding mogul. A few years ago, she even toyed with the idea of changing her last name to Sugarpova, an indication of how deep her entrepreneurial spirit ran.

But with the disclosure this week that Sharapova had failed a drug test, that future is now in doubt. On Monday, the 28-year-old Russian held a press conference to announce that she had tested positive for a recently banned substance called meldonium, which is used to treat heart problems but also has certain performance-enhancing properties. The test was administered at the Australian Open in January, where Sharapova lost in the quarterfinals to her longtime nemesis Serena Williams. (This year’s Australian Open was overshadowed by allegations of rampant match-fixing in professional tennis. But as I noted at the time, many tennis enthusiasts and people involved with the game professionally were more afraid of a doping scandal than cheating.) Sharapova is now facing a lengthy ban from the game, and Porsche and two other sponsors, Nike and Tag Heuer, have suspended their relationships with her. Tag Heuer, which has backed Sharapova since 2004, says it will not renew its soon-to-expire contract with her.

Sharapova is nothing if not P.R. savvy, and she clearly thought that breaking the news herself would be a way of “controlling the narrative,” as flacks like to put it. Her camp billed the press conference, held in Los Angeles, as a “major announcement,” which led many people to assume that Sharapova, who has been battling an arm injury since last summer (she had played just three tournaments in the last eight months), would be retiring. Instead, she stunned everyone by disclosing the failed drug test. She claimed that it was an innocent mistake: she said that on the recommendation of her physician, she had been taking meldonium for the last decade because of abnormal E.K.G. readings and concerns about possibly incipient diabetes. She noted that meldonium, which is known to increase oxygen uptake and endurance, had only become a banned substance as of January 1 of this year and said that she had neglected to open the e-mail from the World Anti-Doping Agency (WADA) announcing that it was now prohibited.

While some commentators were quick to praise Sharapova for her forthrightness, her explanation was quickly picked apart. In response to Sharapova’s disclosure, the Latvian pharmaceutical company that produces meldonium said patients usually require a four-to-six-week course of treatment that may be repeated twice a year, not a decade of continuous use. It turns out, too, that WADA had issued multiple notifications about the pending ban on meldonium; given Sharapova’s obsessive attention to detail and the size and competence of the team around her, it seems odd that she would have been unaware of this. There hasn’t been a lot of support for her from fellow players. Sharapova has always been a somewhat aloof figure on the tour, which hasn’t endeared her to competitors. Still, the relative silence has been conspicuous. Nike, Porsche, and Tag Heuer chose not to remain silent, and it is expected that other sponsors—a list that includes American Express, Avon, Evian, and Head—will follow suit.

It is a stunning downfall for the world’s highest-paid female athlete and tennis’s most bankable star (combined, her endorsement deals earned Sharapova more than $20 million annually). It is claimed that the speed with which her sponsors moved to distance themselves from Sharapova speaks to the hyper-zealous brand management that corporations now practice. But last year, Nike signed a deal with Justin Gatlin, the American sprinter who has twice been suspended for doping, which suggests that things are not so cut-and-dry. Without meaning to imply that Sharapova is in any way being victimized here, it seems reasonable to assume that her age—she turns 29 in a few weeks—and her diminished prospects (she hasn’t won a tournament in a year, she won her last major two years ago, and she has slipped to seventh in the rankings) may have made her expendable in a way that she perhaps wasn’t three or four years ago. Sharapova is looking at a lengthy suspension—anywhere from one to four years. With her game already in decline, the enforced absence could effectively mark the end of her career.

This is not the first time that a major tennis star has been caught using a banned substance. In 1997, Andre Agassi tested positive for crystal methamphetamine. The Association of Tennis Professionals (A.T.P.), the governing body for the men’s game, accepted Agassi’s claim that he had taken it by accident (as Agassi admitted in his memoir, that wasn’t true) and kept the failed test secret. In recent years, tennis authorities have been criticized for being insufficiently vigilant about doping, and there has been concern that they were more interested in protecting the biggest names, and the sport’s reputation, than in rooting out drug use. The Agassi episode was the oft-cited precedent. Now, with Sharapova’s outing, tennis officials can claim with some justification that no one is untouchable. But out-of-competition testing, which is the most important component of an anti-doping regime, is still woefully inadequate. And at the risk of ending on a slightly cynical note, it is fair to wonder if here, too, Sharapova’s age and declining game conspired against her. Would tennis officials have been so quick to make an example of Sharapova if she were five years younger and at the pinnacle of her career?