Rusalka, Royal Opera House, London

You think you know Dvorak, you think you are familiar with the genial symphonist - purveyor of springy slavonic dances and New World adventures. And then you hear Rusalka - the first great opera of the 20th century (1900) - and you begin to wonder if any composer ever tapped into a richer and deeper vein of lyricism in a single work. Wagner, of course, casts his long, tall shadow over this enchanted natureworld. But the singing voice is unmistakably Dvorak's, and it's a sad song he sings.

No one knows it better than Sir Charles Mackerras. His sensational recording of the work endorsed the superstar status of Renée Fleming in the title role - and she was with us again for this "concert" staging by the Royal Opera. But all ears were on Sir Charles in the pit. His decision to keep his orchestra there rather than to relocate them to the stage was a wise one. The sound radiates through the house better from the audience side of the proscenium and an open stage backing on to a simple cyclorama whose changing colour can reflect the human and the spirit world of this "lyric fairytale" is all you need to make magic. Sir Charles and the Royal Opera Orchestra made magic. It wasn't just the accenting and inflection that was so authentic but the timbres - the woody quality of the winds, the oaky clarinets, the dappled horns. Well met by moonlight, all of them. And where the strings kicked up a furiant for cavorting wood nymphs, there was fire in every bow-arm.

The cast were almost entirely off- books, freeing them to move and interact in ways that just aren't possible behind music stands. Of course, the Royal Opera should have their own production of the piece. Then again, who would not fear comparison with the staging that first revived this great work's fortunes in the UK: David Pountney's inspired realisation for English National Opera in 1983. Many of his images flickered through my mind like an old movie as this cast went about its business. They were, for the most part, pretty strong. There were stylish Wood Nymphs - Sally Matthews, Ailish Tynan (who took the song prize at this year's BBC Singer of the World), and Tove Dahlberg; a handy double act of Donald Maxwell and Martina Bauerova as Forester and Kitchen Boy; an imperious Foreign Princess in Eva Urbanova; an authoritative if not so vocally imposing Spirit of the Lake in Franz Hawlata; and a Jezibaba from the Kirov Opera's great mezzo Larissa Diadkova whose threatening chest tones (as in "don't point that voice at me") lent new meaning to her entrance line: "Who dares to wake me before sunrise?"

As for Renée Fleming, for whom Rusalka has become such a signature role, the feeling still exists for me that outside of a recording studio it's a bit on the heavy side for her. The big moments do thrill, the top notes are money notes, no question. But I worry about her "chesting" for dramatic effect and although no one croons quite like Fleming - not least by the light of the silvery moon - there are limits to how attenuated the melodic line can become before vanishing altogether. Still, she is a star performer who must have been disconcerted that, on this occasion, her Prince did not come. Sergei Larin was frankly inadequate in a role demanding a heroic tenor with a facility for bel canto - a bit of a contradiction in terms. But that's what Dvorak demands as his hero begs for the kiss that will happily despatch him to eternity. It was left to Mackerras to complete the transfiguration of Dvorak's most benevolent music into something darkly ambivalent.

Tonight, 7pm (020-7304 4000)