Watch Justin Timberlake And Jimmy Fallon's 'History of Rap' Reunion

The Fallon regular stopped by premiere week of 'The Tonight Show' to rap, perform and have a good time with his best buddy.

As the closing act to Jimmy Fallon's premiere week of "The Tonight Show," Justin Timberlake definitely saved the best for last. A frequent guest on Jimmy's previous "Late Night" (including a lengthy week of performances known as "Timberweek"), JT had an array of viral fan-favorite sketches to choose from for the "Tonight Show." He and buddy Fallon went with the instant classic, belting out the fifth installment of the "History of Rap."

The duo barely sat for a few impatient moments, skipping the intros and immediately grabbing the mics. They jumped straight into old-school textbook rap, reeling off jams from LL Cool J, Salt 'N Pepa and the Beastie Boys before launching into familiar retro favorites like the "Fresh Prince" theme song, which perhaps nodded to Will Smith's appearance on the premiere episode earlier that week.

Rolling into modern song selections, the two included Kendrick Lamar's "Swimming Pools (Drank)," Drake's "Started From the Bottom" and a mash-up of Jay Z's "99 Problems" and Ludacris' "Move Bitch (Get Out The Way)." They finished the performance by jumping into the studio audience for RUN DMC and Aerosmith's "Walk This Way" as the grand finale.

Back at the desk, they joked with best-friend banter. Justin admitted he had tried Mucinex to battle the recent illness that caused him to postpone a tour date at Madison Square Garden in New York earlier this week. "Now I'm on straight up hardcore things. We're not going full Rob Ford, but probably enough that you shouldn't be on national television," he joked.

JT also didn't let the hour pass without resurfacing his particularly spot-on impression of Fallon. After mimicking the "Tonight Show" host with repeated phrases like "so good" and "my man" on set (energetic hand gestures included) the two relived a clip from the "Saturday Night Live" Christmas episode, where JT's impression of Jimmy left them both cracking up on camera.

Justin closed the evening as the musical guest, grabbing the guitar and singing "Not a Bad Thing" backed by The Tennessee Kids. While Fallon may introduce his close friend as "the biggest pop star on the planet," it's nice to see that through improvising song lyrics, slipping into Camp Winnipesaukee voices and rapping the oldies the two really are just Best Friends Forever.