Guilherme Assunção was working in the sandwich area at family-owned market Russo’s when he noticed that singers were setting up sound equipment over by the poinsettias. Russo’s, near Boston, had hired the singers to bring some holiday spirit to the store.

Assunção wandered over as the performers were testing the setup and offered to sing a song for them to equalize the sound. Okay, the guy said, as long as you can sing.

“Yeah, I can sing,” said Assunção, who was recently promoted from dishwasher and had worked at the market for four months.

With that, his operatic rendition of “O Holy Night” filled the store, stunning customers and passersby. Some grabbed their phones to record the moment.

“People told me the whole store stopped — even the employees stopped to watch me,” said Assunção, 23. “A lot of people were filming in front of me.”

The owner’s daughter called over her father, who was impressed, and asked whether Assunção would sing with the other performers the following day, Saturday. Assunção ended up singing with the group Sunday, too, bringing soulful renditions of “Hallelujah” and Andrea Bocelli’s “Con Te Partirò” to customers at the store in Watertown, Mass.

Videos of Assunção’s grocery store serenades quickly made their way around the Internet, turning him into a singing sensation across the country and in his native Brazil.

“I called my family. They are going crazy,” Assunção said. “They are super happy.”

Before Assunção moved to the United States two and a half years ago, he starred in a high school production of “The Phantom of the Opera.” He was already in college, but his former high school let him join the production, which was so popular it would seat an audience of about 5,000 people nightly, Assunção said.

He moved to Utah to pursue dreams of a singing career but first took English classes and enrolled in some college courses. It became too expensive, and he heard about a job at a market near Boston and moved to Massachusetts. Russo’s specializes in produce, flowers, cheese and catering.

Assunção started taking business classes at a technical school but didn’t give up on his goal of someday attending Berklee College of Music and becoming a professional singer.

“I try to keep my feet on the ground, but I dream about a career as a musician here,” he said.

Though for today, he’ll be making sandwich deliveries for Russo’s.

“They’re amazing people here,” he added. “They’ve all been really supportive.”

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