The two performed Lambert’s hit, "Something Bad," together, and judging by the posts the country stars' friends and fans later shared on social media, it was quite the memorable collaboration.
Former American Idol contestant Lauren Alaina -- who also performed at the event -- shared an adorable photo of she and Underwood smiling at one another on stage.
"This is a moment I will NEVER forget," the "Road Less Traveled" singer wrote. "Sharing the stage with @carrieunderwood is absolutely magical. She is such an amazing inspiration for all of us girls with dreams. Thank you for being proof that women can achieve their goals. Probably my new favorite picture."
The official Twitter page for Circus Circus -- a casino hotel in Sin City -- tweeted their excitement, too.
Another performer and Idol alum, Kellie Pickler, was equally as impressed by Underwood's set.
The night's all-female lineup also included performances by Martina McBride, Maddie & Tae, Cam, Kelleigh Bannen, Brandy Clark and Jana Kramer. Kelsea Ballerini also performed and was presented with her ACM award for New Female Vocalist by ACM CEO Bob Romeo.
The night ended with a grand finale performance of "Before He Cheats," where Underwood was joined by some of the aforementioned ladies, including Kramer who posted a brief video of the "girl power" moment on Instagram.
On Saturday, Alaina posted a snap of herself posing alongside her fellow country gals Pickler, Underwood, Bannen and Kramer.
"Still reeling from last night," she captioned the pic, which was re-posted by Underwood. "It was such a honor to share the stage with such empowering women. How is this my job?!"
Underwood also took to social media to thank Lambert for joining her onstage for the "epic" surprise. Lambert shared the same photo on Sunday, captioning it, "Thanks @carrieunderwood for having me Friday night!"