She moved to Germany to study with Max Lorenz for a short period before making her professional opera debut in 1964 at the Hagen Theatre as Elisabeth in Wagner's Tannhäuser.
She replaced an ailing singer at the last minute in the role of Brünnhilde in Wagner's Ring Cycle at the Bayreuth Festspielhaus.
Also in 1974 she sang the Ring Cycle again at the Opéra National de Lyon and appeared at the Teatro di San Carlo.
In 1976, Knie abruptly left a production of The Ring after clashes with producer Patrice Chéreau and poor audience reaction to the then-controversial updating of the opera's setting to the Industrial Revolution.
High points in her stage career include Senta in The Flying Dutchman, Elsa in Lohengrin, Sieglinde and several of the Valkyries in Die Walküre, Donna Anna in Mozart's Don Giovanni, Elettra in Mozart's Idomeneo, the Marschallin in Strauss's Der Rosenkavalier, Liza in Tschaikowsky's The Queen of Spades, and Leonore in both La forza del destino and Il trovatore, among other roles.