The former American Idol star was performing the pair's collaboration 'Remind Me' at the Bridgestone Arena when Paisley's voice was suddenly heard singing from backstage.
Underwood looked visibly shocked and stopped mid-song when the star walked out to join her, and she hugged him before continuing with the track.
Paisley subsequently mentioned the surprise in a message to Underwood on Twitter.com, writing:
@carrieunderwood howdy. Gotcha. :)— Brad Paisley (@BradPaisley) September 24, 2012
His friend replied:
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