Simona Halep said she felt "confused and betrayed" after being handed a provisional suspension for failing a drug test.
The International Tennis Integrity Agency announced on Friday the 31-year-old had tested positive for roxadustat, a prohibited substance, after providing two samples while competing at the US Open in August.
She was notified of a rule violation on October 7 and now faces a fight to avoid a ban from the sport.
In a statement, the ITIA said: "While provisionally suspended, the player is ineligible to compete in or attend any sanctioned tennis event organised by the governing bodies of the sport."
Swiftly responding, former French Open and Wimbledon champion Halep denied all knowledge of taking the substance and said the sanction was the "biggest shock" of her life.
"Today begins the hardest match of my life: a fight for the truth," she posted on social media.
"I have been notified that I tested positive for a substance called roxadustat in an extremely low quantity, which came as the biggest shock of my life.
"Throughout my whole career, the idea of cheating never even crossed my mind once, as it is totally against all the values I have been educated with. Facing such an unfair situation, I feel completely confused and betrayed.
"I will fight until the end to prove that I never knowingly took any prohibited substance and I have faith that sooner or later, the truth will come out.
"It's not about the titles or the money. It's about honour, and the love story I have developed with the game of tennis over the last 25 years."
The 31-year-old former world number one announced in September she would miss the rest of the season after undergoing nose surgery, with it unclear how long her recovery would take.