He was born in
Pisino, Istria (now Pazin,
Croatia), the second of the four sons of Giovanni Battista (1852-1933) and Amelia Ivancich (1861-1942). His mother was an Istrian Italian native of Pisino. His father was originally from Borgo Sacco, near
Rovereto, but worked as a teacher of classical
philology in the high schools of Pisino and Capodistria (now Koper,
Slovenia). In 1892 he obtained the chair at the high school of Rovereto and returned to Trentino, bringing the family with him. As a result, Filzi began his high school studies in Koper and finished them brilliantly in Rovereto in 1902. He came into contact with the
irredentist circles of Trentino in 1901–1903. In 1904, at the inauguration of the Italian law faculty of the
University of Innsbruck, there had been clashes instigated by the Germans that had caused a death, several injuries and numerous arrests among the Italians, including
Cesare Battisti; following these events, Filzi was head of the Rovereto protest movement. In the same year, he was conscripted, and placed in the 4th hunting regiment of Salzburg of the Austria-Hungarian Empire. In November he ended up under investigation on charges of having favoured the desertion of an Italian comrade; he was acquitted but was dismissed as "politically suspect" at the time of discharge. In the following years he was recalled three times, as usual, for military exercises and on one of these occasions he challenged an officer to a duel who had pronounced insults against Italy; only the intervention of the commander averted the clash. In 1905, in the presence of some Treviso gymnasts visiting Rovereto, he recited an impassioned speech against the Austro-Hungarian empire and promised his commitment to the cause of the Italians in the unredeemed lands. In the meantime he attended university studies, enrolling at the same time in Graz at the faculty of law and in Trieste at the "Revoltella" commercial school. He took an active part in the National League, the Society of Trentino students and the Giovine Trieste. In November 1906 he went with his brother Ezio to Graz to join the Italian students who, asking for more government concessions in school, had blocked university activities. Both were injured in clashes with German ethnic elements. After graduating in law from the University of Graz in 1910, he returned first to Trieste and then to Rovereto, where he devoted himself to being a lawyer at the law firm of Antonio Piscel. ==Military career==