Maria Sharapova dropped a bombshell on Monday, announcing at a press conference that she failed a drug test at the Australian Open in January.

The tennis star admitted she has been taking Mildronate, which is also known as Meldonium, since 2006, and it was recently added to the banned substance list unbeknownst to her, according to The New York Times. Sharapova was informed of the failed test by the International Tennis Federation.

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Sharapova, 28, said she received an email in December from the World Anti-Doping Agency, but didn't look at an attachment that included the updated list of banned substances.

The Russian tennis star has won five Grand Slam singles titles, but hasn't played since losing to Serena Williams in the quarterfinals of the Australian Open back in January.

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Sharapova's press conference initially led to rumors that she'd be retiring, but she quickly refuted that by dropping the shocking revelation about the test.

"I received a letter from the ITF a few days ago," she said via MarketWatch.com. "I will be working with them."

Sharapova was contrite, taking full blame for her wrongdoing.

"I take great responsibility in my job every day and I made a huge mistake and I let my fans down and let the sport down that I've been playing since age 4 and that I love so deeply," she said.

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