Don Carlo, Royal Opera

An operatic truism still doing the rounds declares that for Verdi's Il trovatore you need four of the greatest singers in the world. For Don Carlo, his biggest opus in every way, you need six. Nicholas Hytner's Covent Garden staging hits the mark third time around with five, the exception being a very honourable replacement for what would have been an interesting piece of casting.

Even the production, revived here by Paul Higgins, seems more fine-tuned than I remember to the scope and symmetries of the drama. Schiller's Shakespearean royal epic as translated by Verdi into grand opera remains a model of its kind for the balance of the private and the public, the head and the heart, melodrama and truthfulness. There's enough material for at least three strong operas. One would be sustained by the love triangle between Elisabetta di Valois, Don Carlo - the man she loves - and his father Philip II, the king she's forced to marry, with complications added by the Princess Eboli, who loves the infante and is seduced by the king. A second could be devoted to freedom-fighter Rodrigo, Marquis of Posa. And the stifling air in the scene between Philip and the Grand Inquisitor points the way to a new drama of politics versus religion.

With telling clarity, the Hytner view binds it all together and now seems to me to highlight how everyone at the Spanish court, with the exception of hapless infante Carlo, holds the key to power one minute only to be stricken the next. Swooning or dying characters held in desperate embraces embody the failure of a dream.

Seen from the balcony Bob Crowley's designs, meshing handsome period costume with more symbolic settings of oppressive black against gaudy oranges and reds, interact more cleary with the blocking of characters to show their closeness or their distance in relation to each other. Only one scene still falters visually - the biggest, the auto-da-fé; it's surely time to remove the obstructive semi-circular wall draped with the bloody head of Christ, but there it remains, cramping the style of the Les Mis crowd (the Royal Opera Chorus in flaming form).

Otherwise, the gripping continuity of the drama in the great scenes - the garden outside the monastery, Philip's study and the last farewell at San Yuste - depend on the principal singers' skill. As Verdi unfolds one great aria or confrontation after another, we're left weak-kneed by the achievements of this cast in terms of line, colour and feeling. Jonas Kaufmann takes some time to warm in the opening Fontainebleau act - we're talking the five-act Italian Don Carlo of 1886 - since he's hampered by that bottled backward placement which can strangulate the sound; nor are there sunshine and roses in the voice to characterise the love-hopeful innocent.

Come the monastery bonding-duet with Mariusz Kwiecien's Rodrigo (pictured above), he's already rising to top form. Kwiecien is a singer surely at his absolute peak, musically sensitive enough to tune to his tenor but also the ideal Verdi baritone in legato line, as his usually not much noticed address to Elisabetta in the following scene limpidly demonstrates. After a rather wild Veil Song from Béatrice Uria-Monzon's Eboli, more supple of stage movement than of voice, one of the great sequences in all Verdi never falters.

It's now apparent that in Anja Harteros we have another ideal Verdian: floating a line with luminous assurance, forceful in dramatic exclamation and perfectly at ease in her body, fully revealing the pain and difficulty with which regal composure is achieved. Ferruccio Furlanetto's Philip (pictured below with Harteros) is already a known quantity, but he has added to the depths of a tyrant only Verdi could make sympathetic. The beginnings are already there when a sob, a crack in the voice reveals his anguish to Rodrigo, the son he would have wanted.

Prefaced by Pappano's typically human ROH orchestral cello, Furlanetto takes us straight to the heart of the king's interior anguish in the study scene. The ensuing psychic battle with the formidable if physically frail Grand Inquisitor of Eric Halfvarson - announced by Director of Opera Kasper Holten as ailing, though you wouldn't have known it - matches inky-black voices and baleful orchestral lower brass. A third bass, Robert Lloyd as the mysterious monk, is in much better death's-head form than when I last heard him in the role, and two young singers make their mark in bit parts, promising tenor and Jette Parker Young Artist Pablo Bernsch as the Count of Lerma and Susana Gaspar, an appropriately Heavenly Voice.

Pappano's care with the unusual colour of so many chords, the inflections of the woodwind, sorrowful companions to the star-crossed would-be lovers, go beyond the usual call of duty. Equally admirable is the space he gives his singers to negotiate the larger phrases, the constant rubato he uses to go with them at every point.

In any other cast, Furlanetto's "Ella giammai m'amò" would be the absolute highlight of the evening. Here it simply marked the start of the musical-dramatic game-upping. A great quartet, a slight dip perhaps in Uria-Monzon's "O don fatale" - but again, decent work from a singer of more generalised Verdian skills than the others whom you'd still be glad to see in most productions - before more superb, long-phrased cantabile singing from Kwiecien in Rodrigo's hour of death.

And then, that final act. I've never heard more perfect handling of the insane demands Verdi puts upon his soprano in her big, summing-up aria, and I bet you that those elders who grumble about a past age of great singing never have either. Harteros's compass from that ringing or ethereal top down to a chest voice more impressive than that of the evening's mezzo seems rock solid, her variation in tone-colour for each of the debts to the past the Queen is now putting behind her simply astonishing.

Kaufmann joins her to pull out the stops for his huge phrases before the final duet, and then the atmosphere becomes even more rareified. They sing, on the brink of audibility, about meeting in a better world; in our own, opera doesn't get any better than this. Let's hope the magic lasts when Lianna Haroutounian takes over from Harteros in two performances' time. Five stars, in the meanwhile, for five stars (and one more for the conductor). What a way to crown Verdi bicentenary year.

As news comes in that Harteros has had to cancel her remaining two performances owing to acute tonsillitis, here's some small compensation for those with tickets: her Bavarian State Opera performance of the last-act aria "Tu che le vanità". Not as perfect as on Saturday night, but outstanding all the same

And the final duet with Kaufmann, from the same production

• All performances sold out. Lianna Haroutounian takes over from Harteros from 15 May
• Read about Anja Harteros's 2008 Royal Opera debut on David Nice's blog