Strauss and his librettist Hofmannsthal must have felt very daring when, in the prologue to Ariadne, they decided to lift the lid on the clashing egos and mean machinations behind the scenes in the opera world. But truth proved more riveting than fiction when the Royal Opera revived Christof Loy's modern-dress staging of this work in 2004. The American soprano Deborah Voigt, scheduled to sing the title role, was humiliatingly sacked by Covent Garden when (as she put it) her “big hips” proved unable to fit into the svelte black frock that Loy specified.

If I were Voigt I would have turned my back forever on such a sizeist institution. But all credit to the lass. Few people can have put their bodies through such drastic reconstruction for the sake of art. After a gastric bypass she has shed more pounds than it would be chivalrous to quantify, and now returns in the role from which she was ousted five years ago.

In style, too. Her supertanker voice takes a while to settle. One early high note goes alarmingly astray. But once she gets bewitched, bothered and bewildered, in the sumptuous 15-minute duet where Bacchus (robust Robert Dean Smith) turns the whining, pining Ariadne into an ecstatic lover, she is magnificent. And the new compact curves look good too.

That's one major feature of this revival. The other is Gillian Keith's dazzling debut on the Covent Garden stage as Zerbinetta, the good-time showgirl who is as easy-virtued as Ariadne is staunchly monogamist. No need for any nip-and-tuck surgery on Ms Keith. She must have the trimmest figure and best legs in opera, and she flashes plenty of both as her troupe of dissolute lowlifes (Markus Werba outstanding as a rough-trade Harlequin) crashes into the highbrow opera planned by Kristine Jepson's touchingly naïve Composer.

But Keith, who sounds underpowered in the Prologue, also rises with terrific verve to the challenge of Zerbinetta's fiendish coloratura aria, gradually turning the trills and cascades from expressions of self-mockery into self-loathing. In Loy's neatly conceived staging she ends the opera as Ariadne begins it: slumped despondently in a corner.

Mark Elder - Sir Mark, after last week's honours list - paces the work beautifully, and is rewarded with eloquent playing, especially in the chamber-music passages. Thomas Allen looks aptly harassed as the Music Master; Alan Oke is a properly raddled Dancing Master; Anita Watson, Sarah Castle and Anna Leese make a well-matched trio of nymphs. All worth a visit, not least because you can then quip that you've seen the one opera that ain't even started till the fat lady slims.

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