In 1946 Della Casa sang the part of Zdenka in Richard Strauss 's opera Arabella in ZÃ 1\/4 rich , and again the following year when she sang for the first time at the Salzburg Festival where she was to sing many times during her career . After the performance , Richard Strauss himself said : `` That girl will be the Arabella someday ! '' Della Casa sang the part of Zdenka in the performance of Richard Strauss ' Arabella at Zurich Municipal Opera House to her revered soprano Maria Cebotari 's Arabella in 1946 .