Simona Halep, who has been suspended since October due to a positive test for a banned substance, faces a second doping accusation.
Reuters reported that the latest charge comes from her Athlete Biological Passport that showed irregularities in her profile.
"We understand that today's announcement adds complexity to an already high-profile situation. ... We have remained committed to engaging with Ms. Halep in an empathetic, efficient and timely manner," Nicole Sapstead, senior director for anti-doping at the International Tennis Integrity Agency, said in a statement.
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"It would be inappropriate for us to comment on specifics until the conclusion of the process, but we will continue to engage with the Sport Resolutions independent tribunal and Ms. Halep's representatives as expeditiously as possible."
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Halep had previously been suspended due to a positive test for roxadustat at the U.S. Open last year, which she has denied knowingly taking.
The Romanian has been ranked No. 1 in the world a total of 64 weeks in her career and won the 2018 French Open and 2019 Wimbledon championships. She reached the Australian Open final in 2018, and her best finish at the U.S. Open is the 2015 semifinal.
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She is currently ranked 30th in the world.