Mozart for the eyes as well as the ears

For reasons typographical you may never see the name of Anne Teresa De Keersmaeker splashed across posters or put up in lights, but those in the know need little prompting to turn out for Belgium's first lady of contemporary dance. On Wednesday she and her company, Rosas, currently in residence at the Theatre de la Monnaie in Brussels, played their only British date this year, a fitting finale for the Turning World season. While ballet has long been pulling audiences to the South Bank's biggest venue, this was a first for modern dance. The Festival Hall was full and buzzing and, on the strength of it, more is bound to follow.

Keersmaeker has one enviable advantage over leading British choreographers: great funding. For her current show, Mozart, she brings a 39-piece baroque orchestra (the excellent Anima Eterna), three world-class singers, and a custom-made dance-floor, which alone is rumoured to have cost pounds 15, 000. But the vast, circular, Come Dancing parquet is not what 2,000 people have come to see. Keersmaeker has a reputation for ambitious, intricate, highly evolved works that seek new ways of fusing with classical music, performing a two-way function as a dance-spectacle and concert. Toccata explored the music of JS Bach; Stella brought Ligeti to a wider public; Mikrokosmos did the same for Bartok. But Mozart ... is there anything to say about him that a play, a film, and a bicentenary year of back-to-back Figaros and Nachtmusiks didn't?

Of course, but Keersmaeker demands our faith and a solid two hours (no interval) to find it. The first check to expectation is her designer's idea of rococo: this is 1780s mixed with 1930s glam, a ballroom-world of Bakelite lights, curly wrought-iron chairs and coy sopranos in droopy frocks. One of the three, from the middle of the parquet, launches into one of the better known of Mozart's dozen or so concert arias, "Un moto di gioia". A male dancer saunters on as she sings, flamboyant in frock- coat and breeches, and tries to distract her, prancing and pratfalling, flirting and flouncing, flicking up his heels and his coat-tails in a parody of period glee. At the final, triumphant top note, he plucks her up in his arms and makes off with her, squealing. So far, so entertaining. Wolfgang the wag was a hoot.

Things begin to look more like hard work when Keersmaeker's disingenuous scheme takes hold. The composer wrote 10 concert arias performable by soprano, and, doggedly, the remaining nine follow, the soloists taking turns to deliver these intense, substantial mini-dramas (stupendously, I must add) while a company of 13 dances round them. That Mozart's rigorous art can withstand such treatment is never in question. Whether it's appropriate is the point. That's where faith comes in: only after the first hour or so do accumulated riches unfold.

The choreographer's responses float freely between a literal correspondence with the lyrics, biographical reference to Mozart, and pure, 18-carat musical instinct. In a song about the blindness of love, a man leads his sightless ballroom partner perilously close to the edge of the stage, so close that she can't fail to register his cruelty. Another has girls dressed a la Marie Antoinette manically scampering on all fours - a hilarious nod to WA's kinky doggy-games with his sister.

Dress and undress make a continuous sub-theme: period furbelows matched with Belgian-designer chic; the fetish-frisson of half-clothed juxta- posed with full fig; the eroticism of a gentleman delicately removing his partner's shoes and pinning up her ruched skirts for dance (or what?). Sex, playful and profound, runs through the work like a silken thread, unravelling suddenly in a vocalised swoon as six female bodies keel over in a faint, or couples hurtle across the floor in deft whole-body rolls.

Most intriguing of all is the way Keersmaeker insinuates her way to the core of the musical feeling. A seated dancer makes contact with the stage as if its surface is wired for exquisite sensation. The effect is akin to the most poignant aural experience. Another takes an asymmetrical, feral, all-fours pose, twitching almost imperceptibly in a way that - I cannot say how or why - heightens the tortuous workings of Mozart's harmony as it unfolds. The climax of the work, pianissimo and in the dark, is not so much physical as metaphysical. To a muted rendering on fortepiano of the sublime Rondo in A minor, the company pares down the act of dancing to a lilting walk, the off-beat creak of their insteps merging with the rondo's pulse in a way that suggests infinity. They tell me Theatre de la Monnaie, Brussels, is very close to the Eurostar. I would go to the ends of the earth.