Maria Sharapova announced Monday that she failed a drug test at the Australian Open in January.
She said she received a letter from the International Tennis Federation on Wednesday but has not yet found out what her penalty will be. The ITF confirmed the failed drug test and said Sharapova would be provisionally suspended until the case is finalized.
"I take great responsibility and professionalism in my career every day," Sharapova said. "I made a huge mistake. I let my fans down. I let my sport down."
Sharapova's lawyer, John Haggerty, described the range of penalties to ESPN's Jim Caple.
"When someone intentionally takes banned substance for performance-enhancing purposes, there is a four-year ban," he said. "If it is not done intentionally, the top end of the ban is two years. If there are mitigating circumstances -- as I strongly believe there are here -- there can be an even greater reduction, including eliminating sanctions."
Sharapova, 28, said she had been taking the drug mildronate, also known as meldonium, for 10 years to address a number of health issues, including low magnesium levels. Meldonium is a blood-flow-promoting drug banned because it helps oxygen uptake and endurance.
Sharapova said she had been getting the flu every couple of months, had irregular EKGs and had evidence of diabetes, which runs in her family. She said she did not realize that mildronate and meldonium are the same drug and was not aware when rules changed Jan. 1 to make the drug illegal. She said that when the World Anti-Doping Agency sent an email about changes to the banned list in December, she did not click on the link to see that the substance had been added to the list.
"I know with this that I face consequences," Sharapova said. "I don't want to end my career this way, and I really hope I'm given another chance to play this game."
Jeopardizing her sponsorships stands as one of those consequences. On Monday night, Nike became her first sponsor to suspend their relationship.
"We are saddened and surprised by the news about Maria Sharapova," Nike said in a statement. "We have decided to suspend our relationship with Maria while the investigation continues. We will continue to monitor the situation."
Sharapova extended her Nike deal in 2010. The eight-year pact was reportedly worth as much as $70 million, including royalties.
Swiss watch brand Tag Heuer said early Tuesday that it was ending its relationship with Sharapova.
"Maria Sharapova was under contract with TAG Heuer until December 31th, 2015. We had been in talks to extend our collaboration. In view of the current situation, the Swiss watch brand has suspended negociations, and has decided not to renew the contract with Mrs. Sharapova," the company said in a statement.
Sharapova had partnered with the brand since 2004.
Meldonium is manufactured in Latvia. It is used in Lithuania and the Russian Federation but is not approved by the U.S. Food and Drug Administration. A number of athletes have tested positive for the drug since Jan. 1: Abeba Aregawi (Ethiopian-born, Swedish nationality, 1,500-meter world champion), Eduard Vorganov (Russian cyclist), Endeshaw Negesse (Ethiopian marathoner), Ekaterina Bobrova (Russian ice dancer) and Olga Abramova (Ukranian biathlete).
Sharapova addressed rampant speculation that she would announce her retirement.
"If I was going to announce my retirement, it wouldn't be in a downtown Los Angeles hotel with this fairly ugly carpet," she said.
Sharapova said she tested positive in an in-competition test at the Australian Open, where she lost to Serena Williams in the quarterfinals on Jan. 26. Sharapova hasn't played since, as she is recovering from a left-forearm injury.
Haggerty tried to explain how Sharapova could be unaware that she was taking a banned substance.
"She is very organized, and she takes her career very seriously, which is why when she first started to take this back in 2006, she made sure it was approved, that it wasn't on the banned list and checked in future years," Haggerty said. "It was just an honest mistake. We're not making excuses, but because she had taken it for so many years and it was OK year after year, it just fell off the radar."
Haggerty said meldonium was banned because "it does appear that some athletes have used it in some circumstances in bigger dosages to gain some competitive advantage. Maria was taking a dosage that was significantly lower."
Haggerty said that when Sharapova received the letter from the ITF, she was "shocked, completely stunned. She takes great pride in her integrity and the way she has approached the game all these years. She was surprised but immediately wanted to come forward and take full responsibility and acknowledge what happened."
Sharapova has 35 career singles titles and more than $36 million in career earnings. She is currently No. 7 in the WTA rankings after playing just three tournaments and the Fed Cup final in the past eight months because of injuries. She dropped out of the upcoming BNP Paribas Open in Indian Wells on Thursday, citing injury.
Sharapova is thought to be the world's highest-paid female athlete thanks to her extensive business ventures and endorsement deals. Forbes estimated her earnings at $29.5 million for 2015, with $23 million from off-the-court ventures.
Sharapova has been among the best players of her generation since she burst onto the scene as a 17-year-old Wimbledon champion in 2004, when she beat Williams in the final. She won the US Open in 2006 and the Australian Open in 2008 before completing her career Grand Slam with French Open titles in 2012 and 2014.
She became the world's top-ranked player in August 2005 and has held the ranking five times for 21 weeks in her career. Although she is nowhere close to matching Williams' 21 Grand Slam titles, her five Grand Slam titles rank behind those of only Serena and Venus Williams among active players.
Sharapova was born in Russia and lived briefly in southern Sochi before moving to Florida as a child to begin her tennis career. She lives primarily in the Los Angeles area now.
WTA CEO Steve Simon released a statement after Sharapova's announcement.
"I am very saddened to hear this news about Maria," he said. "Maria is a leader, and I have always known her to be a woman of great integrity. Nevertheless, 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 Program and its standard procedures. The WTA will support the decisions reached through this process."
The Associated Press and ESPN's Darren Rovell contributed to this report.