Maria Sharapova announced Monday that she failed a drug test at the Australian Open earlier this year.

Sharapova received a letter from the International Tennis Foundation several days ago informing her that she had failed the test for a substance that had been legal until Jan. 1 of this year. Sharapova did not indicate what the penalties would be or how long she might be away from the game of tennis.

She had begun taking the substance Meldonium in 2006 to aid in a variety of health problems. However, the drug was added to a banned list starting at the beginning of 2016. Sharapova conceded that she did not look at the list of newly banned substances for 2016 from the World Anti-Doping Association, and thus indicated that she failed the drug test because of ignorance.

"I don't want to end my career this way," Sharapova said. "I really hope I will be given another chance to play this game."

Meldonium aids in athletes' endurance and rehabilitation. Doctors prescribe it to treat ischemia, a lack of blood flow, but it can give athletes an increased ability for exercise. It had been on WADA's watch list in 2015, and was formally banned at the beginning of this year after WADA indicated there was “evidence of its use by athletes with the intention of enhancing performance." On Dec. 22, 2015, WADA sent an email indicating what drugs would be banned starting in 2016; Sharapova conceded that she received but did not read that email.

You can read more about Medonium here on Yahoo News.

Sharapova has not competed since January's Australian Open, where she fell to Serena Williams in the quarterfinals.The five-time Grand Slam champion has suffered a rash of injuries over the last few years, including one that forced her withdrawal from last year's U.S. Open and, more recently, an arm injury that resulted in her pulling out of the BNP Paribas Open at Indian Wells in California.

Sharapova, 28, is currently ranked 7th in the world. A former World No. 1, her 35 singles titles and five Grand Slams ranked third among active players behind Venus and Serena Williams. Sharapova won the French Open twice and Wimbledon, the Australian Open, and the U.S. Open once apiece, making her one of only ten women, and the only Russian woman, to hold the career Grand Slam.

Sharapova won at least one singles title every year from 2003 to 2015, a remarkable run. She has played in 733 career matches and won 80 percent of them for $36 million in career on-court earnings. She's diversified herself off the court, parlaying her tennis fame into modeling and fashion success. She has been the highest-paid female athlete in the world 11 years running.

Widespread speculation had indicated that Sharapova would retire, but in a bit of humor at an otherwise somber press conference, Sharapova indicated that if she was to announce her retirement, it would not be at "a hotel in downtown Los Angeles with a fairly ugly carpet."

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Jay Busbee is a writer for Yahoo Sports and the author of EARNHARDT NATION. Contact him at jay.busbee@yahoo.com or find him on Twitter or on Facebook.