On a Friday night late last month, Romeo Santos paces the stage of Madison Square Garden practically on tiptoe. Crooning in pillowy Spanglish, he blows kisses to the nosebleeds as if picking an apple from a tree in reverse. Even his smile seems to curl in slow motion.

He’s savoring it. It’s the 30-year-old bachata star’s third sold-out gig there in February. It’s also his longest, spanning more than three hours — past midnight, past union curfew and into a dreamy delirium where the wail of young women begins to resemble a heavenly dolphin choir.

The screaming peaks in the homestretch as Santos points to one fan in the audience (“I love you . . . ”) and her date (“ . . . I hate him”). He pulls another breathless admirer up onstage. Then he does something with his hips that we can’t really talk about here.

He wasn’t always this bold. Two mornings later at a midtown hotel, Santos remembers the teenage years he spent singing at house parties, unable to pull his eyes from the carpet. “Every time a song would be over, there would be silence,” he says. “I was extremely shy.”

Today, Santos can’t afford to be. Breaking from his trailblazing group Aventura, his solo debut, “Formula Vol. 1,” spent three months at the top of Billboard’s Latin albums chart thanks to a push from Sony, which turned Shakira and Ricky Martin into household names. Now, Santos has his own ABC sitcom in development and can already be spotted during commercial breaks on Univision, pitching cellphones for AT&T.

But where Shakira and Martin crossed over to non-Latino audiences by singing in English, Santos sings almost entirely in Spanish and refuses to stray far from the love songs of bachata, a genre born in the plantations of the Dominican Republic.

As he prepares to hit the road — there’s a Patriot Center concert March 9 — the Bronx-born “king of bachata” says he isn’t worried about crossing over to mainstream America: “I want them to cross over to my world.”

“Them” is big enough to include Usher, who lends his voice to Santos’s biggest hit, “Promise.” Instead of a halfway rendez-vous in some predictable dance-pop DMZ, Santos and the R&B star duet over bachata’s signature sounds — rippling bongo drums, a grinding guira, and sugary-sharp notes plucked from acoustic guitars.

Usher, himself, materialized to sing “Promise” during the grand finale at Madison Square Garden, capping off a night of guest performances that included reggaeton heavyweights Wisin y Yandel and Sean “Diddy” Combs. “He’s the newest, biggest superstar in the game,” Combs declared from the stage. “And he’s gonna be here a long time!”

The three-night run at Madison Square Garden illustrated how Santos is expanding his reach without really moving his feet. With nods to R&B, hip-hop and reggaeton, the concerts also included appearances from Prince Royce, a young star undeniably forged in Aventura’s mold, as well as Luis Vargas and Anthony Santos, two long-beefing bachata legends that Santos invited to bury the hatchet by appearing on his new album.

Santos grew up on their songs alongside three older siblings in the shadow of Yankee Stadium. (Born Anthony Santos, he eventually took the stage name “Romeo” to avoid being mistaken for the veteran bachatero Anthony Santos.) Raised by his Dominican dad and Puerto Rican mom, the rest of Santos’s musical education came from the streets.

“What didn’t I hear?” Santos says of his youth in the Bronx. “Open a window, I’d be listening on one corner to salsa, on the other corner you’d hear bachata, merengue.”

He also heard the creamy tones of R&B and the charismatic cadences of hip-hop, which informed the high, sweet vocal delivery that belies Santos’s sandpapery speaking voice and brawny frame.

He got his start singing with his cousin, Henry Santos, in the choir at St. Thomas Aquinas. But faith didn’t bring him through the church doors. “I joined the church choir because there were these two hot chicks,” Santos says. “Then people started giving me compliments. ‘You really have a good voice.’ Really? I just joined the choir for these girls.”

By the time he was 16, he had formed Aventura with Henry, and his friends Lenny and Max Santos, two brothers who are unrelated to Romeo. His parents disapproved of his musical ambitions from the start. “I never got permission to be an artist,” he says. “But they also felt like this was a stage. ‘He’ll get over it — he wanted to be an astronaut last year.’ But I wasn’t gonna stop doing this.”

That determination — and the group’s eagerness to tweak tradition — would eventually help sell millions of albums. “In bachata, you had these guys that used to wear suits and had a really traditional style,” Santos says. “We looked different. Baggy jeans. We had the Spanglish going on, and I knew that was going to work to our advantage.”

It was the sound, too. The hip-hop, R&B and merengue that flowed through Santos’s window as a kid flowed into the group’s music, earning the scorn of purists who dismissed Aventura as a boy band but the adoration of a new generation of multicultural Latinos.

“The big Latin pop stars have been imports, historically,” Billboard’s Leila Cobo says of Aventura’s connection with young American audiences. Aventura “are from here, and their audience is from here. The best parallel, in my mind, is somebody like Selena who was from Texas and spoke to people who were like her. . . . Romeo appealed to an audience that was like him and didn’t see other people like them up on the stage.”

The group’s popularity surged in 2009 with the release of its sixth album,“The Last,” which spent 23 weeks at the top of Billboard’s Latin albums chart. The foursome performed at the White House that October, and in January of 2010, they kicked off a sold-out, four-night stand at Madison Square Garden. Down the street, Lady Gaga was launching a sold-out, four-night stand of her own — at Radio City Music Hall.

Santos says that Aventura plans to record another album together next year and has been wrestling rumors to the contrary since the hiatus was announced. Fans questioned the split, worried that the frontman might follow in the footsteps of Marc Anthony or Enrique Iglesias.

“People were like, ‘Pfft — That guy’s gonna start doing pop, man,’ ” says Santos. “I wanted my fans to not feel betrayed.”

So even though he’ll sing a few songs in English for an expanded edition of “Formula” that will be released later this year, Santos remains loyal to his sound, hoping to expand his brand on television. He’ll get to work on the still-untitled comedy pilot once he returns from tour. The show, which is being produced by Will Smith’s Overbrook Entertainment, will center around a traditional Dominican American father who doesn’t understand his son’s greater ambitions. (Santos says he can relate.)

And it’ll all be set in the Bronx — a place where the singer can no longer set foot today. If he did, heads would turn, fans would swarm, traffic would jam, life would tangle itself into a knot.

Could he imagine his career swelling to the point where that might happen on Anystreet, U.S.A.?

“I just want people to know me,” Santos says, still sounding a little shy, but not like he doesn’t want it to come true.

Romeo Santos

performs March 9 at the Patriot Center.