About three years ago, songwriter Jonathan Singleton was at a bar in Nashville when a singer he didn’t recognize took the stage. Suddenly, the crowd started buzzing, as though a superstar had just arrived. “What is happening?” Singleton asked his friends, but they didn’t know, either. Singleton caught the singer’s name and then discreetly took out his phone to Google: Who is “Luke Combs”?

Singleton had no idea that soon it would be tough to find a country music listener who didn’t know the answer. But at that moment, as he heard Combs’ powerhouse voice belt out a song called “Hurricane,” he knew he wanted to work with the guy. A few months later, Singleton signed Combs to a publishing deal with Big Machine Music. And “Hurricane” would eventually go triple platinum.

Fast-forward to now, and Combs, 28, has shot to stardom so quickly that it’s startling to almost everyone, including him. After he landed a record deal with Sony Music’s Columbia Nashville in fall 2016, his first four singles reached No. 1 on the radio, with the fifth (“Beautiful Crazy”) expected to soon hit the top of the chart. His debut record, “This One’s For You,” was the highest-selling country album of 2018. Most of the dates on his first arena headlining tour are already sold out. On Sunday he’ll be in the national spotlight at the Grammy Awards, where he’s nominated for best new artist.

“It’s really the fastest thing I’ve ever seen in this town,” said Singleton, who has written hits for Tim McGraw, Billy Currington, Gary Allan and others. The thing he remembers most from that first night is how loudly fans sang along to Combs’ songs, which had found early popularity online. “He really hit the nail on the head of what they were looking for.”

It has all led to an enduring mystery that everyone in Nashville wants to solve — how other singers can replicate Combs’ success.

“People always ask me, ‘What’s the secret?’?” Combs said in a phone interview, laughing. “If I knew what it was, I would be bottling it and selling it instead of what I’m doing now.”

Pressed for an answer, Combs will offer that it’s a combination of things: Hard work. Personal sacrifice. Luck. Timing. Surrounding yourself with trustworthy people. Writing songs that you would want to hear on the radio.

Others point to Combs’ traditional ’90s country-influenced sound, which, combined with modern production, struck a chord with fans, or suggest that some singers just have that intangible X-factor. Rob Williford, Combs’ bandleader and frequent songwriting partner, has another theory.

“I genuinely think for him, it’s nothing but the lack of trying to have a brand. It’s literally authenticity at its core. It’s unabashed, ‘This is who I am.’ It’s the lyric, it’s the melodies, it’s the production, it’s the guy that goes onstage in a black PFG shirt every night,” Williford said of Combs’ signature fishing gear apparel. “It’s 100 percent authentic. You can’t fool people when it comes to that.”

When Combs moved to Nashville in September 2014, a “brand” was the furthest thing from his mind. A North Carolina native, Combs had enjoyed singing in middle school and high school but first picked up a guitar in college at Appalachian State University about 2011. He started out playing gigs at the same bar where he worked in Boone, N.C., and persuaded his boss to charge $1 a ticket. He made $200 one night, and it dawned on him that maybe, one day, he could earn a living playing music.

He scraped together enough cash to record two EPs and posted performance videos to Facebook and Vine. He racked up thousands of followers and began playing every bar in the area, sometimes with a crowd of a few hundred.

When Combs made the permanent leap to Nashville, he continued to tour in the Southeast, and earned a following. He co-wrote songs with other newcomers who didn’t have connections with established songwriters.

“We got in there and were, like, ‘Hey, we don’t really know how the game works,’?” said Ray Fulcher, who co-wrote eight songs on Combs’ debut album. “Instead of figuring that out, we said, ‘Let’s just write some songs that we would want to listen to.’ “

The strategy paid off, especially with “Hurricane,” which Combs wrote with Thomas Archer and Taylor Phillips. Even though he didn’t think it was “outstandingly memorable,” he uploaded it to iTunes; it sold nearly 15,000 copies in a week. The song, describing the jolt of unexpectedly seeing an ex, became Combs’ first hit.

Although the plan was to slowly and deliberately build his fan base, his singles turned into streaming and radio smashes and sold faster than expected: “When It Rains It Pours,” about a guy who gets dumped and then has a streak of good luck; “One Number Away,” about the temptation to call an ex; “She Got the Best Of Me,” the aftermath of a rough breakup; and “Beautiful Crazy,” a much more optimistic tune, inspired by his girlfriend-turned-recent fiancee, Nicole Hocking. Last week, Combs became the first solo artist since Tim McGraw in 2000 to have two songs simultaneously in the Top 10 of the Billboard Country Airplay chart. (“Welcome to the club buddy,” McGraw tweeted to Combs.)

But even as Combs has gone from playing for 250 people in bars to selling out 12,500-seat arenas, he’s determined not to forget his working-class roots, his true connection with his die-hard fans. He still wears his black PFG shirt onstage every night, and although that resulted in an endorsement deal with Columbia Sportswear, his fans just see the guy with a beard and black shirt and hat and boots, dressed very similarly to them. Singleton said they call those fans “Luke-alikes.”

“When those fans are there, they see themselves up there,” Fulcher said, adding that Combs often tells his audience, “If I can do this, you can literally do whatever your dream is.”