The doctor at the centre of the Operacion Puerto doping case, Eufemiano Fuentes, has revealed that a footballer was among his patients prior to his arrest in 2006.
Fuentes, his sister and three cycling coaches are currently on trial in Spain charged with breaking public health laws.
Operacion Puerto investigated allegations of doping in sport and the suspected supply of several high-profile cyclists with EPO and other performance-enhancing drugs.
Police working on the operation arrested Fuentes in 2006 and seized 200 blood bags from his premises.
"I worked with all kinds of athletes back in 2006. Cyclists, footballers and athletes," said Fuentes during Tuesday's hearing.
"I was not working with a cycling team at the time. I worked with individual athletes, with an athlete, with a footballer, with a boxer ..."
The prosecutors must prove that the defendants' actions risked the lives of athletes. Fuentes claims he was helping the athletes' recovery from anaemia rather than enhancing their performance illegally.
The hearing is expected to continue for the next six weeks. Fuentes could face up to two years in prison if he is found guilty of endangering the health of the people he treated.