World number 10 Guillermo Canas has been banned for two years after testing positive for a banned diuretic.
Canas reached the last eight of this year's French Open
The 27-year-old Argentine, winner of six ATP tour events in his career, tested positive in February at the Mexican Open in Acapulco.
Canas appealed against the finding to an independent Tennis Anti-Doping Program tribunal but it confirmed the results of the dope test.
He now intends to make a final appeal at the Court of Arbitration for Sport.
Canas said: "I am a professional player and it's difficult to know what happened. But the truth is this, we don't know what happened."
And Canas argued the ATP had made the wrong decision.
"The ATP have made a mistake in my case," said Canas.
"It was a lapse by the whole system at the Acapulco tournament.
"It was people from the tournament who went to buy me the medicines with the guarantees of the ATP.
"I never in my life thought they would give me the maximum sentence.
"My career is not going to end, that's for sure. I'm suffering a great injustice."
The panel deducted $276,070 in prize money from Canas, as well as his singles and doubles ranking points.
He will be eligible to return to tennis on 11 July 2007.