Watch 'Voice' Hopeful Brennley Brown's Stunning Patty Griffin Cover

Teenage singer from Gwen Stefani's team takes on Griffin's "Up to the Mountain" in knockout round performance

Brennley Brown performs Patty Griffin's "Up to the Mountain" on 'The Voice.'

It's hard being a teenager on The Voice, but with 15-year-old Brennley Brown's polished vocals, her age all but fades away. What emerges is a powerful, memorable performer that is shaping up to be one of the most formidable competitors on the show's 12th season.

For her knockout round performance against 14-year-old Aaliyah Rose, Brown chose Patty Griffin's "Up to the Mountain (MLK Song)," popularized by Kelly Clarkson but first recorded by Solomon Burke in 2006. In her rendition, Brown lent the powerful anthem her distinctively country twang, even eliciting a stupefied "What?" from notoriously tough coach Alicia Keys.

During rehearsal, however, things did not run quite as smoothly. Coach Gwen Stefani urged Brown to show more of her personal spirit and emotional depth. She responded with an impassioned performance, featuring beautifully executed runs and a restrained use of her head voice. Coupled with her larger-than-life stage presence and booming lower register, Brown captured the song's resilient, gospel-like power with maturity well beyond her years. The performance is currently one of YouTube's top trending videos.

The coaches had a hard time expressing their reactions, with Adam Levine saying, "Brennley, you gave a Grammy performance. And you're 15, which is just freakish." Brown's former coach Blake Shelton conceded, "It still hurts me to see her on somebody else's team." Ultimately, the decision came down to Stefani, who said, "You were like this unbelievable surprise I didn't know I was gonna get. You impact people because of your spirit. It was incredible." Torn, Stefani said she was going to make the decision solely based on that specific knockout performance and announced Brown as winner. Then came a not-so-subtle reminder from host Carson Daly that Rose – who sang Meghan Trainor's "Like I'm Gonna Lose You" – was available to steal, and Shelton pounded his buzzer to secure Rose for his team.

In Team Blake news from last night, 30-year-old Terry Pinkard, who goes by TSoul, performed Otis Redding's soul classic "These Arms of Mine," for his battle round (watch below). His swagger and smooth, confident vocals carried him to victory over opponent Enid Ortiz, who took on Adele's "When We Were Young."

The Voice live performances start Monday, April 10th at 8:00 p.m. ET on NBC.