They performed a medley of rap hits on "Late Night With Jimmy Fallon" Wednesday night, and the clip is becoming a viral hit.
Fallon and Timberlake started off with Sugarhill Gang's classic "Rapper's Delight" and then performed jams from artists including the Beastie Boys, the Notorious B.I.G., Jay-Z and Eminem.
Fallon, 36, said he's surprised by the success of the skit.
"We never know what's going to viral or get picked up, and this one's beyond what we thought it would do," he said Thursday in a phone interview with The Associated Press.
Though the clip looks all fun, he said, he took the time to get it right before taking the stage.
"You know I really needed to practice. You know I can't rap," said Fallon, who also performed songs by Missy Elliott, Tupac, Kanye West and Snoop Dogg. "I was in my apartment till 2 in the morning the night before doing (the) Soulja Boy (dance) and my wife was like, 'You're crazy!'"
Though Fallon needed to rehearse, he said, Timberlake was obviously a natural: "(He) knew all of his parts, all his rhythm was amazing, so much better than mine. Mine was so bad, my gosh, (I thought), 'I'm going to screw this up,'" Fallon said.
The performance was backed by the house band for the NBC show, the Grammy-winning hip-hop group The Roots. Fallon said that much of the success goes to the rap band and that the skit was a homage to today's generation.
"The thing that makes our show kind of different is that we embrace hip-hop. ... We're that generation where hip-hop is what we grew on," Fallon said.
He also said that office morale is at a high: "Everyone's so smiley and running around the office like very confident today, because everyone sees the blogs and gets the e-mails. It just puts everyone in a great mood."