Watch Carrie Underwood and Jimmy Fallon Duet 'Islands in the Stream' on 'The Tonight Show'

Carrie Underwood's appearance on The Tonight Show Starring Jimmy Fallon featured one incredible karaoke performance of Kenny Rogers and Dolly Parton's "Islands in the Stream."

Underwood was featured on Fallon's recent historic episode of The Tonight Show in Central Park. T-Mobile took the late night host out of his home studio and into the scenic New York City park earlier this month, where he put on his usual show before a live audience of thousands in a concert-like outdoor setting. Underwood was on the show that night promoting her new album, Cry Pretty, but a newly-released video shows her impromptu karaoke performance with Fallon.

The show's official YouTube account posted the video on Sunday. It features Underwood and Fallon singing the classic country ballad, "Islands in the Stream," originally performed by Kenny Rogers and Dolly Parton in 1983. The two had fun with the song, making it clear that it was an off-the-cuff rendition with no previous rehearsal.

While Fallon laughed his way through a few lines, Underwood kept it mostly professional, at least for the first half.

"Here's the key change," she called out to the band halfway through the song. They slid easily into the new key, while Underwood came in a little low. "Okay, I'll come in now," she said. After one line she added "that's really low."

She took it in stride, however, laughing with Fallon, and belting out the next few lines at a powerful volume.

The karaoke performance was left out of the original broadcast, meant as a treat for the live crowd only. However, now that almost two weeks have passed since the massive show, it seems NBC wants to let some more content trickle out. According to viewers who attended, there were other Karaoke performances as well, including Underwood helping Fallon perform Post Malone's "Psycho."

Underwood's interview that night was equally charming. In it, she shared the struggles of a professional singer with an outgoing child. She admitted to Fallon that she "judges" her 3-year-old's singing.

"Is that wrong?" she asked, joking, "I'm like judging my 3 1/2-year-old child to see if he can sing." She added that she would be fine if Isaiah grew up and had no interest in music.

"If he is not musically inclined, that is fine," Underwood said of her son. "I'm excited to find out what he loves and is passionate about and is good at and I will support that."

"But I really want him to sing," she added.