It's no secret that Justin Timberlake is a triple-threat as a singer, movie star and TV actor. He proved it again last week on "Saturday Night Live"
 and he's doing his best to cement that reputation with his week-long guest stint
 on "Late Night with Jimmy Fallon."

Tuesday night's barbershop quartet take on "SexyBack" was cute, but Timberlake went all-in on Wednesday night with a pair of oddball skits and the mesmerizing TV debut of "Let The Groove Get In," from his upcoming The 20/20 Experience album.

The song, which is a hypnotic, if meandering and overlong funk workout on the album, got the full treatment on Fallon's show, with house band the Roots joining JT's already hefty backing band, the Tennessee Kids for a show stopping performance. With more than 20 musicians packed onto the stage, the song's hip-shaking samba rhythms were instantly infectious thanks to a dozen percussion instruments shaking and rattling along to the hypnotic refrain, "Are you comfortable right there, right there/Let the groove get in there, there, right there." The horns, guitars and keyboards kicked in and Timberlake laid his smooth vocals over the massive groove, daring the audience to stay in their seats as he rolled his hips from side-to-side. The tune built and changed over the course of more than six minutes, mutating from a two-step samba into a Michael Jackson falsetto breakdown and an Earth, Wind & Fire-style breezy pop funk tune (complete with lyrical shout out to "Shining Star").

Earlier in the show, Timberlake and Fallon joined Doobie Brothers legend Michael McDonald for a soul-dipped cover of "Row, Row, Row Your Boat," with JT and Jimmy sporting white wigs and beards in homage to McDonald's signature look.

The singer also worked alongside another legend, "The Incredible Burt Wonderstone" star Steve Carell, in a lengthy bit that was allegedly based on "Jacob's Patience," a Canadian soap opera that gave Carell and Fallon their starts in the business.

Timberlake and Carell played men's shop employees helping Fallon buy a, yes, "Suit & Tie," only, everyone in the bit had to use mannequin hands instead of human appendages. Things got really weird when Timberlake had to measure Fallon's inseam. And JT could hardly contain his laughter when Carell pointed to exquisite suits from the Joseph Fatone and Lance Bass collections. "It's been in the closet for years ... we finally decided to take it out," Carell said of the Bass pieces as Timberlake bit his lip about the joke about his out-and-proud former 'NSYNC bandmate.

"Can you sing about how quickly you work?" Fallon asked pal Timberlake. "Oh you want me to sing?" Justin deadpanned back, rolling his eyes and halfheartedly crooning, "I will make you a suit and tie/I'mma sew it up for you tonight/Let me measure your inseam."