Giselle, Royal Opera House, London

There is nothing like a threat of cancellation for sharpening the expectation of pleasure. Only days ago word went out that Alina Cojocaru, the Royal Ballet's 20-year-old Romanian star, lately the company's premier ticket-shifter, was to withdraw from her entire run of Giselle. An old metatarsal injury has been playing up, and doctors were advising rest – just like Giselle's old mum in the ballet, wagging her finger at the frisky girl who insists on joining in the village dances, heedless of the heart complaint that threatens her life. Instead, Cojocaru went against advice and played Giselle. And it was the audience who landed up with the heart condition – the happy condition of being full to bursting.

What's immediately special about Cojocaru's Giselle is the way she inhabits the role so simply and without fuss. Where some ballerinas draw attention to the ballet's Romantic-periodness, delivering the hallowed steps as if set between quotation marks, Cojocaru seems to live right inside it. Thus every step has an impulsive freshness, springing directly from her situation. For two hours you forget about dance technique, and you forget about acting. Cojocaru is Giselle – and the effect is so apparently artless and utterly convincing that during the mad scene I felt an urge to rush on stage and cuddle her.

Cojocaru's artistry plumbs even greater depths in Act II where – as the spirit wraith pleading for her lover to be spared the wrath of the wilis – she distils her dancing into a stream of pure feeling. Refinement of technique is her argument for clemency, and Cojocaru bends every last sinew to her supernatural task in a way that seems less of a personal statement, more a generic embrace of every Giselle who has gone before. Technically, there are wonderful things here – Cojocaru's quick, softly cushioned jump, an arabesque so pure it breaks your heart – but by the end, the dancing has become so rarefied that just the poise of her head says as much.

Johan Kobborg must take huge credit for making this intensity possible. As a partner, his attentiveness and acute musical timing allow Cojocaru to seem to waft not only on drifts of air, but on the shape of each phrase of the score. As a dance-actor, he is the perfect match: subtle, reactive, showing the cogs of a mind at work. And clarity is the prominent quality of his solo dancing, too, Albrecht's punishing strings of entrechats drawing gasps from the house as their buoyancy and crispness increased with repetition.

When two such talents come together, the combustive effects touch everyone on stage. The whole company took fire on opening night, with notable contributions from Muriel Valtat (whose motherly warning in mime was the clearest I've seen) and Zenaida Yanowksy as a truly terrifying Queen of the Wilis. Conductor Emanuel Plasson powered through Adolph Adam's hoary music as if it were a taut, new film score – packed with suspense.

j.gilbert@independent.co.uk

'Giselle': in rep until 16 April, ROH (020 7304 4000)