Renee Fleming continues to surpass herself. The American soprano has been hailed for her creamy tone and easy lyricism and for the warm, distinctly personal elegance of her stage presence. Still, I doubt that even her most ardent admirers could have listened to Fleming's Tuesday night performance of Richard Strauss's "Daphne" at the Kennedy Center Concert Hall without some amazement -- who knew she had the power and calm to manage this crushingly difficult, unearthly beautiful music?

To be sure, she has recorded "Daphne" in a two-CD set newly released on the Decca label. But recordings are not live performances. (Placido Domingo recently recorded Wagner's "Tristan und Isolde" for EMI, but he did so one act at a time, knowing that taking on the whole thing in one evening might be a voice wrecker.) And one of the principal difficulties of "Daphne" is that it must never sound difficult. It is a fiercely strenuous opera about composure, a sensually blooming exploration of chastity.

This one-act, 100-minute "bucolic tragedy" -- which was presented in concert without scenery or dramatic realization of any kind -- marks the moment when Strauss began to find his late voice. He was in his early seventies during the creation of "Daphne" (begun in 1936, finished in 1937, presented for the first time in 1938) and nothing much more was expected from him. Many young composers had written him off as a dinosaur -- as an apostate modernist who had startled the world half a century before but then turned his back on his own innovations and devoted his energies instead to overripe and overdone romantic spectacle.

There was some truth to those charges. Strauss's middle period remains his least satisfying. But the late works are something else again. Throughout these compositions (the operas "Daphne" and "Capriccio," the Oboe Concerto, "Metamorphosen" for 23 solo strings and the peerless "Four Last Songs") he created music of brilliant autumnal radiance -- not unlike the weather we've been experiencing the past few days: precious, transitory, glorious even in its decay.

"Daphne," with a libretto by Joseph Gregor, is based on the classical myth of a wood nymph who spurns the advances of Apollo and is transformed into a tree. This poses considerable staging problems, which is only one of the reasons the opera is best known from recordings. Another reason might be that, as with the "Four Last Songs," it is nearly impossible for a soprano to project such cool, sweetly elevated music over the sound of a huge orchestra without breaking a sweat -- and a single drop would spoil everything. In these works, Strauss demands a combination of Mozartean purity and Wagnerian force, a stark contradiction that is most easily reconciled with recording technology.

Yet Fleming met most of the score's challenges and met them with a freshness becoming to a gentle nymph living long ago on an unspoiled planet. At the opera's close, she sang the newly deciduous Daphne's final wordless murmur with her back to the audience, an appropriate distancing technique that worked.

The cast was generally strong, but every singer was presented with some problems by the wildly impractical music Strauss created for this opera. There are two parts for high tenor; the difficulties of the passages sung by Daphne's friend Leukippos (portrayed ardently and intelligently, but with a voice a size too small, by Roberto Sacca) are spectacular but as child's play compared with the workout Strauss reserves for Apollo. Jon Fredric West sang this immense, ringing, stratospheric music with such unfettered, red-faced intensity that one half-feared for his health (he took several swigs from a glass of water during rests in his final declamation).

Meanwhile, contralto Anna Larsson, in the role of Gaia, was called upon to sing notes as unusually low for a woman as some of the music for Apollo and Leukippos is high for a man. Her voice is a distinctive one, with the dark, rich timbre associated with the contralto register but none of the typical heaviness. Bass-baritone Robert Holl made a fine, booming Peneios. As two servant girls, Julia Kleiter and Susanne Bernhard intertwined their voices with playful ecstasy, as though they were two of Wagner's Rhinemaidens who had been transported into another myth.

A huge element of the evening's success was the sweeping and detailed conducting of Semyon Bychkov and the lush, intricately variegated, supremely smooth playing he elicited from the WDR Symphony Orchestra of Cologne; they drowned out some of the singers some of the time, but that is mostly Strauss's fault. The transformation scene was played slowly and prayerfully, as though it were the last and most beautiful music in the world and should never be allowed to end.

Roberto Sacca portrays Leukippos ardently and Renee Fleming's performance soars in Strauss's "Daphne" at the Kennedy Center.