Prima Donna, Sadler's Wells

Why write gluey pastiche Massenet and Puccini when you could compose as your flamboyant self? Why collaborate on a cliché-ridden French text when your song lyrics declare themselves so piquantly in English? Rufus Wainwright must have his own reasons for concocting a fantasy of what opera might, or used to, be.

That was largely due to the dignity of a fine production team, very different from the one behind last year's Manchester world premiere, and a top-notch quartet of singers led by a great if underrated singing actress, soprano Janis Kelly. She's lived on stage the agony of traviata Violetta's fading charms and the gaze into the mirror of Richard Strauss's Marschallin, facing the ageing process and the loss of her young lover. Yet this character, Régine Saint Laurent, is a strange and dubious archetype of a disintegrating operatic icon, not a person; any of the human questions about what becomes of the woman once la voce deserts the artist simply aren't explored.

The first act, which attempts to set up the crisis, is a musical-dramatic quagmire, and none of director Tim Albery's habitual clarity can make it move any more naturally. Puccini, whose perfect sense of dramatic timing might have been more useful to Wainwright than his harmonic flavourings, would never have started an opera with the bittersweet dawn introduction we have here; surely he would have saved it for a last-act prelude, having plunged us into the bustle of the big night on which Régine is about to fail, here very much in the past. Wainwright and co-librettist Bernadette Colomine show us Régine already declined six years down the line; she can only retreat into an operatic dreamworld. We could have skipped with no loss the pallid duet with sympathetic maid Marie (Rebecca Bottone) and the hiss-boo bullying of ambitious Philippe (veteran Jonathan Summers, as a character left to his own brutish devices), and started with curtain-up on the interview with our newly-Deneuved Prima Donna about to be conducted by a young journalist.

Up to this point, Wainwright has resisted all but a phrase or two of his signature style, lapsing from romantic opera to the kind of musiquette churned out by minor French composers in the 1920s. Now, as Colin Ainsworth's ringing tenor André Letourneur sings of his own potential opera career that never materialised and proves - quelle surprise - he can play the hero to Régine's greatest triumph as Aliénor d'Acquitaine, a haunting ostinato takes over. Soon the orchestral colours, well handled by the Royal Ballet Sinfonia under Robert Houssart, begin to parallel a typically wacky Wainwright arrangement and we breathlessly wait for the hit aria and duet from the iconic opera.

They don't quite deliver, but we do remember them when they return in a focal dream sequence in Act Two. It starts with a dramatically redundant but rather attractive ditty for Marie - Wainwright in folk vein, Bottone managing stratospheric writing to match Ainsworth's gamely taken challenges, though neither younger singer is comfortable with the lower-register demands - before Régine takes true centre stage. Again, Wainwright's purely instrumental ambition is surprising, as the diva's vocalises pass to celesta and on to feathery strings (as far as I can work out, Wainwright did the orchestration himself with "assistance"). It's now that the discreetly crumbling Parisian apartment (designed by the usually phantasmagorical Antony McDonald) lends its own eerie, candlelit magic before the windows lift to be replaced by a moon and cypresses and the Aliénor duet has its dream high noon.

Tristan und Isolde it isn't, but Wainwright then moves fluidly to Régine's final breakdown. Kelly resists hamming it up and, with the dangers of a potential Rosenkavalier trio duly avoided - our journalist tells her he has a date with his fiancée, Sophie (geddit?), and she bows out with a Marschallin-like grace hardly merited by the situation - glides to the final number as Régine sees the ephemera of life in a Parisian firework display. Simple in its range and effect, the only "aria" in the show to sound like Wainwright at his authentic best - words excepted - it's beautifully served by La Kelly and by the fading lighting of Thomas Hase, a key player in the evening's success. The illusion holds to the blackout. Maybe it was all smoke and mirrors; maybe that was the point. If Régine is trying to tell us, like Shakespeare's Parolles, "simply the thing I am shall make me live", that would be a relief after all the kitschy flim-flam. Unlike the premiere's director, Daniel Kramer, Albery at least gives us that more human option. Anyway, Rufus seems to have had his vision, and without, it seems, an eye to commercial success. In that, at least, he remains idiosyncratically on course.

• Three more performances of Prima Donna at Sadler's Wells on 14, 16 and 17 April