The opening night selection of this year’s Tribeca Film Festival was another music documentary — “Time Is Illmatic.” So does that mean Robert De Niro has a favorite Nas song? “No,” De Niro said, admitting he wasn’t familiar with the rap star’s lyrics. Then he put his finger to his lips: “Shhh!”

The rest of the crowd practically sung along to “Illmatic,” which tells the story of the groundbreaking 1994 album that put Nas on the map. The Wednesday night screening at the Beacon Theater, owned by Madison Square Garden (a perk of the festival’s recent deal with the entertainment giant), was a double feature of sorts. The film was followed by a nine-song concert from Nas, who was introduced by his friend Alicia Keys, playing the piano.

“I only got involved recently,” Nas said, of the Tribeca-financed film. “But I’m happy I did.” He wouldn’t specific how long he filmed for. “I gave them … more than one day,” he said.

PHOTOS: Nas, Alicia Keys Celebrate Tribeca Film Festival Opening of ‘Time Is Illmatic’ 

First-time director One9 said that the Nas film was a labor of love, which he started working on 10 years ago. “I never dreamed this,” One9 said of his Tribeca debut. And he explained his name as such: “I grew up as a street artist — one is the beginning; nine is completion.”

The film’s credited writer, Erik Parker, added that his job title wasn’t meant to be taken literally. “We did everything from PA work to producing to directing,” Parker said, noting that the film’s budget ranged from “zero to zero to zero.”

“I don’t want say what it wound up being, but we started out of out pocket,” One9 said. “It was never enough.”

Before the screening, Nas’ younger brother Jungle felt nervous about his big-screen debut. “I don’t want nobody’s feelings to get hurt,” Jungle said. “I just told the truth a lot about my life. Nas shouldn’t get mad, and I’m not going to get mad.”

Tribeca co-founder Jane Rosenthal introduced the film along with De Niro. The after-party was held at the midtown’s Providence NYC nightclub, where Nas was whisked upstairs to a VIP room. “I want to thank Robert De Niro,” Nas said earlier on-stage. “He plays me in the movies.”

(Pictured: One9, Nas, and Erik Parker at the Tribeca Film Festival opening night premiere of “Time Is Illmatic”)