Demiddi, who grew up in Rosario, started out as a swimmer, but began rowing at age 16. He developed to become one of the best single sculls rowers in the world, only lacking an Olympic title. He won the World Title in 1970, a Pan American Games gold in 1967, four South-American titles, one European title, 11 Argentine titles as well as the Henley Regatta in 1971. At his three Olympic appearances, he improved every time, going from fourth in 1964 to second in 1972. He took up coaching at Racing Club de la Marina, but declined a position as with the Argentine squad.