Two tenors in the same role at the same time _ what could have been a recipe for disaster turned instead into a delightful operatic performance of Christoph Willibald Gluck's "Alceste," with Joseph Kaiser's sudden voice loss leading to a bit of successful improvisation.

With his final performance Saturday night as artistic director of the Washington National Opera, tenor Placido Domingo ends a 15-year collaboration that transformed what was once a respectable provincial company into what he recently described as "one of the really important ones in the United States and the world."