French Open finalist Mariano Puerta has denied taking any banned substances following reports that he tested positive at Roland Garros this year.
French newspaper L'Equipe said the world number 10 provided a positive test for the stimulant etilifrine.
But Puerta said: "I'm really angry. I've started investigations with my lawyers. There's no truth in it."
Puerta has previously served a nine-month drugs ban so he could face a life ban if the test is confirmed.
A spokesman for the ATP, the governing body of men's tennis, would not confirm the positive test, saying: "The ATP does not discuss doping cases until a decision by an independent tribunal is rendered."
Puerta said that he had not yet been informed of the test.
A statement issued on his behalf said: "All positive alternative explanations for the alleged positive result are urgently being investigated, and he is currently being advised as to the legal and procedural options open to him, in full co-operation with the International Tennis Federation."
Puerta served a suspension in 2003 when he tested positive for the anabolic steroid clenbutherol.
He is one of five Argentine players to have failed drugs tests in the last five years.
Guillermo Canas, Guillermo Coria and Juan Ignacio Chela have all served suspensions, while Martin Rodriquez was docked ATP points and prize money.