In 1989, her final year at the Sofia Conservatory, she and 5 other students were sent to a month-long concert tour of France. The artist agent Luisa Lasser-Petrov made a recording of one of her performances which she sent to Herbert von Karajan, who immediately requested to see her in Salzburg and then in Vienna. They met in Salzburg where she was asked to sing "Agnus Dei" from Bach's Mass in B minor. Karajan wanted to engage her to sing Bach's mass with him at the Salzburg Festival the following year, but died shortly after. The Impresario of the Vienna State Opera, Ioan Holender (then jointly directing with Eberhard Wächter) was persuaded by Lasser-Petrov to hear her auditions and immediately offered her a 2 yrs contract at that house to start in 1991-92 season. When she returned to Bulgaria, she took part in another audition at Stara Zagora Opera where she was heard by Christoph Groszer, the impresario of the Zurich Opera. He also immediately engaged her for his company.
After graduation, Kasarova joined the ensemble at Zurich Opera in 1989 and made her professional stage debut in Wagner's Götterdämmerung as the 2nd Norn and Wellgunde. She soon became a local favorite there with the audience appreciative of her unique vocal timbres, expressive intensity, and virtuoso ability. She also entered and won that year's Neue Stimmen competition. The competition was sponsored by Bertelsmann, which owns BMG Classics label, leading to her exclusive recording contract.