Watch Anna Kendrick steal John Krasinski's girl on 'Lip Sync Battle'

Maybe John Krasinski should have been the sixth member of #NSYNC? #LipSyncBattle

Anna Kendrick has a thing for Emily Blunt, and John Krasinksi -- a.k.a. Jim Halpert -- definitely should have been in a boy band. 

These are just two of the lessons learned from Thursday night's "Lip Sync Battle."

The "Pitch Perfect 2" actress and "The Office" alum faced off in an episode of the viral Spike TV competition, and things got personal. 

Krasinski, who performed Tina Turner's "Proud Mary," also shined as the sixth member of 'N Sync (and a marionette!) in his version of "Bye Bye Bye." He may have needed to ice his knee after a drop, but it was totally worth it.

But Kendrick made a dig at his  personal life when she performed a hit from another boy band -- One Direction's "Steal My Girl" -- that turned into a surprise love letter to Blunt, her "Into the Woods" costar and Krasinski's wife of nearly five years. (Blunt also competed on the show in Anne Hathaway's infamous "Wrecking Ball" episode.)

"This song is from my heart and I'm excited to finally share it with the world," Kendrick admitted before her performance.

Along with the Oscar nominee's broody rendition of the love song, she had a homemade video montage that paid tribute to the British actress. She also accessorized with an Anna-and-Emily T-shirt. 

And the look on Krasinski's face was classic.

"Can I get a close up of that T-shirt?" Krasinski asked. "I'm not gonna lie, it sort of hurts my heart."

When host LL Cool J asked if "The Devil Wears Prada" star was in on the prank, Kendrick wholeheartedly agreed.

"Oh yeah, she knows. She was super excited, she was like, 'I won't tell him,'" Kendrick said in a fake accent. "She was supposed to be here but she's in Dubai. Classic Emily!"

Still, Krasinski got in a final quip and teased Kendrick about her "Cups" song. Check out the banter in the videos above. 

Follow me on Twitter @NardineSaad.

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