Giordani had sung Edgardo in Donizetti's "Lucia di Lammermoor" on Friday, and had not sung Romeo since 2003 in Vienna. But he was willing to give it a try.
So he rushed from his apartment to meet his vocal coach, Bill Schuman, talked with the Met about an hour later and decided to go ahead.
"I was afraid I couldn't remember, but I remembered 95 percent of the score," Giordani said.
Giordani got to the opera house about 5:45 p.m., a little more than two hours before curtain time, was fitted for a costume and went on stage to rehearse with soprano Anna Netrebko. The performance, conducted by Placido Domingo, went off on time.
Giordani was focused intently on the prompter's box and Domingo. Giordani was pleased with his performance.
"Even if I forget words, I know I can ad-lib," Giordani said, "I know I have help."
Raul Melo had been the scheduled cover, but Met general manager Peter Gelb prefers subbing with a star singer if one is available and willing.
Giordani, burlier than most Romeos, said there was an amusing moment during the production's scene in which he and Netrebko were on a bed, suspended above the stage.
"People were laughing because I was moving a little bit more. They were afraid I was going to fall off," Giordani said. "Usually you have a small guy."
Giordani became the third tenor in four performances to sing opposite Netrebko this season, following Roberto Alagna and Joseph Kaiser. Rolando Villazon, who was originally scheduled to sing Romeo at the Met, cancelled before the start of the season. Matthew Polenzani is scheduled later in the run, which extends through Dec. 31.