Sanguineti began his high-level tournament career at the 1954 Buenos Aires Zonal tournament in Mar del Plata, with a fine mid-place (tie for 7-9th place) finish of 10.5/20. He improved the next year, 1955, with an excellent tie for fourth place at the Argentine Championship at Buenos Aires, where he scored 12/19. Next was the very strong Buenos Aires 1955 event, which featured star Grandmasters Borislav Ivkov, Svetozar Gligorić, Herman Pilnik and Laszlo Szabo, and he could only make 7.5/17 for 13th. But he followed this up with a much better result of third at the annual Mar del Plata International of 1956, which often attracted many of the world's best players during the 1950s and 1960s. There he scored 10.5/15 (tie for 3rd-4th).