In a gambit that threatens to disqualify him from Three Tenors status, Placido Domingo has joined the ranks of the baritones. Domingo announced what must surely be his final career move — his 69th birthday fell in January — to an astonished world last year: he was to essay the major role of Simon Boccanegra, the compromised Doge of Genoa in Verdi’s opera of that name.

He duly appeared in the role in Berlin, New York and Milan, and last night saw his keenly awaited baritone debut at Covent Garden. In a sense Domingo is coming full circle: it was as a baritone that he auditioned at the National Opera in Mexico City at 18. And indeed his tenor has always had a baritonal quality: one that makes him better suited for the brooding Siegmund in Wagner’s Die Walküre, for example, than for the overtly virile athleticism of Siegfried.

Now that is turned on its head and we have, in his Boccanegra, a baritone with a distinctly tenorial timbre. He can certainly reach all the notes of the role effortlessly and as he drops into the lower register he exudes the character’s humane, grizzled authority — this is a popular leader with a chequered past.

But then as he rises above the stave, the thrill familiar from his heroic tenor roles is there: now he’s in his comfort zone and it’s a place where he can be depended upon to deliver the goods. In all honesty it wasn’t the greatest Boccanegra ever, but he earned a standing ovation nevertheless. The tenor role of Gabriele Adorno, which Domingo would normally have sung, is taken by Joseph Calleja. There is a trace of a beat in his voice, but his ringing high notes also won the hearts of the audience.

The sepulchral bass of Ferruccio Furlanetto as Fiesco and the baleful baritone of Jonathan Summers as Paolo throw Domingo’s crossover register into relief. Marina Poplavskaya once again shows her huge potential as a Verdian soprano with her heart-warming Amelia.

Antonio Pappano’s conducting is tautly controlled but shot through with flashes of theatrical brilliance. Elijah Moshinsky returned to brush up his straightforward 1991 production, now revived for the fifth time.

On BBC TV (July 10), on a nationwide big screen live relay (July 13), at the Proms and on Radio 3 (July 18). At ROH (020 7304 4000; www.roh.org.uk) to July 15.