The country singer stars as a church youth leader in the upcoming biopic of teenage wave-rider Bethany Hamilton, who refused to quit the sport after a shark bit off her arm.
Underwood was prepared to be underwhelmed by her performance - but admits she was pleasantly surprised with herself.
She tells the Associated Press, "I really liked how everything turned out, including myself. I was really expecting the worst from myself, and I really surprised myself."
But the American Idol winner insists she won't be quitting her day job any time soon.
She adds: "There are people that are absolutely amazing actors and actresses, and I am not one of those, though who's to say one day; you never know, maybe I will be. But I'm definitely new at this and just doing what's fun right now, and I love my music; that's my first love."