In 1966, the New York City Opera revived Handel's then virtually unknown opera seria "Giulio Cesare" (with Norman Treigle as Caesar), and Sills' performance as Cleopatra made her an international opera star. Sills also made her "unofficial" Met debut at a Lewisohn Stadium summer concert performance as Donna Anna in "Don Giovanni", though nothing further came of this other than offers from Rudolf Bing for roles such as Flotow's "Martha". In subsequent seasons at the NYCO, Sills had great successes in the roles of the Queen of Shemakha in Rimsky-Korsakov's "The Golden Cockerel", the title role in "Manon", Donizetti's "Lucia di Lammermoor", and the three female leads Suor Angelica, Giorgetta, and Lauretta in Puccini's trilogy "Il trittico".