Ken Howard, Associated Press
Joseph Kaiser as Romeo and Anna Netrebko as Juliette in the Metropolitan Opera production of Gounod's "Romeo et Juliette."

NEW YORK — At 66, Placido Domingo says he's too old to sing the part of a love-struck teenage Romeo. But he's better suited than anyone for the role of coach and mentor to a young tenor who's considered a budding opera star.

Joseph Kaiser made his Metropolitan Opera debut in Gounod's "Romeo et Juliette," with Domingo — best known as a tenor — conducting the orchestra. Kaiser, a 29-year-old Canadian, was a surprise replacement for famed tenor Rolando Villazon, who canceled six weeks earlier.

Audience expectations were high — possibly of an overnight sensation.

Kaiser entered with a solid, but cautious voice, overshadowed by the volcanic force of nature that is the soprano of Anna Netrebko, opera's hottest female star. He may have been suffering from nerves.

He went on to sing the aria "Ah! leve-toi, soleil" — Romeo's serenade to Juliette, imploring his lover to rise like the sun from her balcony — with more assurance but still held back the kind of full-throttle emotional thrill his talent promises.

He finally let loose, fittingly, after singing "Remonte au ciel, prudence infame!" ("Return to heaven, infamous caution!"), throwing vocal caution to the wind for Romeo's ardent explosion as he fatally knifes Tybalt, the rival who had wanted him dead.

Kaiser displayed fine musicianship all evening, spinning out Gounod's rich melodic lines with a warm, clarion tone that often reached vocal heights with ease, even a high B and C. But he may need some more nights at the nation's premier opera house to reach his peak performance.

Domingo, who had once also sung the role of Romeo, supported his protege's every breath with his baton from the Met orchestra pit. Weeks earlier, in Salzburg, Austria, the master tenor approached the younger artist during a break while Kaiser was performing Tchaikovsky's "Eugene Onegin."

"I said to him, 'What about doing Romeo at the Met?"' Domingo said in an interview with The Associated Press. "I said, 'Your voice sounds wonderful, and I think you can do it.' He has a lyric voice and he has power. I hope a star will be born."

Domingo was already familiar with Kaiser, a 2005 prize winner of the Domingo-run Operalia international voice competition who got to the finals of the Met's National Council Auditions the same year.

But stepping in for Villazon — hailed along with Netrebko as the current golden couple of opera — was a different challenge.

The Montreal native told the AP that Domingo was the finest mentor. "He's like one of those great athletes who makes the very conscious decision to devote the latter stages of his singing career to the next generation, to passing on the tricks of the trade."

The tricks Domingo shared with Kaiser included pacing — and "not playing the tragedy" of the two lovers for the whole opera. "There are so many times when Romeo just stands there and lets her words wash over him," Kaiser said. "He's like, bathing in them. It's the biggest, biggest, biggest turn-on — her saying stuff about him, and about them. Placido said, 'Don't be afraid to smile, you're happy, you're 16. Enjoy!"'

Still, Kaiser added, he knew there would be nervous moments, "When you say, 'Oh God, this is the Met stage, Domingo, Anna Netrebko!"'

Hearing a highly praised new singer live for the first time was tantalizing for the capacity Met audience of nearly 4,000, which rose to applaud at the end while saving its loudest accolades for Netrebko.

Kaiser's remaining two Romeo performances at the Met were sold out. And the buzz around him includes blogging fans who have dubbed him "Hunkentenor" and "Kute Kaiser."

His good looks were very much on display in a scene of writhing passion with Netrebko on a white bed suspended high above the Met stage — a dazzling production effect by Guy Joosten that drew giggles from the audience.

By the end of the three-hour-plus performance, the two soaring voices melted into the physical rapture of a pair of teenagers whose love turned into rebellion against their feuding families in Shakespeare's 14th century Verona.

It's been only two years since Kaiser's voice was first noticed in the music industry, singing the small part of a prisoner in Beethoven's "Fidelio" at Chicago's Lyric Opera. Director Kenneth Branagh quickly chose him to play Tamino in a new film of Mozart's opera "The Magic Flute."

Kaiser replaced Villazon after the 35-year-old Mexican tenor announced in August that he was canceling all performances on doctor's orders; the nature of his illness has not been disclosed.

Two other singers also took over the part of Romeo at the Met — Roberto Alagna, who sang two performances in September, and Matthew Polenzani, scheduled for December.

Netrebko is also a Domingo protegee; the Spaniard has secured a number of engagements for the 36-year-old Russian soprano, who's now the reigning new diva of the early 21st century.

Kaiser is to appear at the Met again in November as Tamino in "The Magic Flute" — originally his Met debut role.

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