Diener appeared as Donna Elvira in Mozart's Don Giovanni at the Aix-en-Provence Festival in 1998. The same year, she appeared as Agathe in Weber's Der Freischütz at the Vienna State Opera. She made her debut at the Bayreuth Festival as Elsa in Lohengrin in 1999 and performed the role the following two years. In Vienna, she also appeared as Elettra in Mozart's Idomeneo in 2000, as Contessa Almaviva in his Le nozze di Figaro in 2002, as Ellen Orford in Britten's Peter Grimes in 2005, and as Elvira, Elsa, and Chrysothemis in Elektra by Richard Strauss in 2007. In 2013, she appeared as Isolde in Wagner's Tristan und Isolde with the Canadian Opera Company in Toronto, a role which she also performed at the Opéra national du Rhin in Strasbourg.
