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Maria Sharapova failed a drug test during the Australian Open.

The former number-one player in the world tested positive for meldonium, a drug that facilitates recovery from injury and, crucially, increases an athlete’s endurance.

Sharapova made the announcement herself during a Monday afternoon press conference. The 28-year-old said that the drug was part of her rehabilitation regimen as she recovered from injuries that have limited her appearances on tour since 2013. She lost to Serena Williams in the Australian Open quarterfinals. 

Meldonium joined the banned substance list on January 1, and Sharapova admitted she had not read the updates to the World Anti-Doping Agency’s list of banned substances. So far it’s unclear what disciplinary measures the International Tennis Federation might hand down.