During his studies in France in 1924, János Hajdú already as a well-known foil and sabre fencer became acquainted with épée fencing. In Paris, he was a student of Lucien Gaudin, later champion of foil and épée at the 1928 Olympics, who encouraged Hajdú to popularize the fencing discipline through national magazines after his returm in Hungary. Hajdú was the organizer of the first Hungarian Championship in épée and, at his initiative, Hungarian team participated in the Amsterdam Olympics. He was the winner of the first Hungarian Championship in épée in 1930 and was still good enough the title, at age 43, in 1947. After his retirement he worked as a sports diplomat and international fencing referee, and as a teacher at the College of Dramatic Arts. He participated in fencing scenes of theater productions and feature films. In 1926 he graduated from Pázmány Péter University with a doctorate in law.