From the Audience to the Stage at the Metropolitan Opera

Hiroyuki Ito for The New York Times The soprano Danielle de Niese performing in Central Park in July.

The soprano Danielle de Niese came for a relaxing evening at the Metropolitan Opera - in the audience â€" with her husband and parents on Tuesday night. Instead, she found herself onstage.

On the morning of the performance, Natalie Dessay told Met officials that she was too ill to sing in Handel’s “Giulio Cesare.” “Symptoms of a cold,” said Peter Gelb, the Met’s general manager. “She could not vocalize properly.”

So Mr. Gelb, knowing that Ms. De Niese was coming to the house anyway, asked her to step in. She arrived at 1 p.m., met with the conductor, Harry Bicket, and went over the choreography. “The costume shop went into overdrive,” Mr. Gelb said.

Mr. Gelb announced the circumstances from the stage, and Ms. De Niese performed to a large ovation.

It is a classic opera substitution story. But the drama had a back story. As it turns out, Mr. Gelb had asked Ms. De Niese to be in town during the early performances of the opera, just in case Ms. Dessay had to cancel.

“Natalie is one of the great artists, but she also is somewhat fragile,” said Mr. Gelb, who pointed out that she has talked about leaving opera and moving back to acting, her early training. “I’m always aware where singers are whenever we have a major new opening or production. Our practice is, whenever possible, when a star is ailing, to get another to step in.”

And the last-minute appearance was not a huge stretch for Ms. De Niese. She originated the role in this production, by David McVicar, at the Glyndebourne Festival and reprised it at the Lyric Opera of Chicago.

The next “Giulio Cesare” performance is on Friday and Ms. Dessay is scheduled to sing then. Ms. De Niese, Mr. Gelb added, will again be available, just in case.