The great turning point in Callas's career occurred in Venice in 1949. She was engaged to sing the role of Brünnhilde in "Die Walküre" at the Teatro la Fenice, when Margherita Carosio, who was engaged to sing Elvira in "I puritani" in the same theatre, fell ill. Unable to find a replacement for Carosio, Serafin told Callas that she would be singing Elvira in six days; when Callas protested that she not only did not know the role, but also had three more Brünnhildes to sing, he told her "I guarantee that you can.". Michael Scott's words, "the notion of any one singer embracing music as divergent in its vocal demands as Wagner's Brünnhilde and Bellini's Elvira in the same career would have been cause enough for surprise; but to attempt to essay them both in the same season seemed like "folie de grandeur"". Before the performance actually took place, one incredulous critic snorted, "We hear that Serafin has agreed to conduct "I puritani" with a dramatic soprano ... When can we expect a new edition of "La traviata" with Gino Bechi's Violetta?" After the performance, one critic wrote, "Even the most sceptical had to acknowledge the miracle that Maria Callas accomplished... the flexibility of her limpid, beautifully poised voice, and her splendid high notes. Her interpretation also has a humanity, warmth and expressiveness that one would search for in vain in the fragile, pellucid coldness of other Elviras." Franco Zeffirelli recalled, "What she did in Venice was really incredible. You need to be familiar with opera to realize the size of her achievement. It was as if someone asked Birgit Nilsson, who is famous for her great Wagnerian voice, to substitute overnight for Beverly Sills, who is one of the great coloratura sopranos of our time."