'The Voice': Girl Named Tom's Tribute to Their Ailing Dad Brings Kelly Clarkson to Tears

The first night of Knockouts concluded with a heartfelt battle -- and a major steal!

The Knockouts are here on season 21 of The Voice, and that means things are getting emotional.

Kelly Clarkson's team was the last to perform on Monday's all-new episode, saving one of the most heartbreaking decisions of the night for episode's end. Kelly pitted impressive vocalist Holly Forbes against harmonizing siblings trio Girl Named Tom, for performances that impressed all four of the coaches.

In rehearsals, Girl Named Tom explained to Kelly that they chose their Knockout song, Glen Campbell's "Wichita Lineman" as a tribute to their father, who is battling terminal cancer, and there wasn't a dry eye in the house as the siblings praised their hard-working patriarch.

"He has such a positive attitude going into every doctor's appointment," said Bekah Grace, as Kelly wiped away tears. "It's been huge for us to see that."

Season 21 Mega Mentor Ed Sheeran offered the group some advice on polishing their impressive harmonies, but also noted how he could relate to the siblings' musical journey. 

"It was my grandmother that was the one that really got me and my brother together," Ed recalled of collaborating with his older brother Matthew, a classical composer. "She was passing away and she said, 'I want to see you work together before I die.' And so that's why we did 'Perfect.'... All those strings are him."

"We're definitely following in Dad's footsteps," Girl Named Tom's Caleb explained. "He wrote songs for each one of us when we were little. The fact that we can be here and call him and share this wonderful news with him everyday, I think it gives us all a lot of life, but it's not easy."

The trio took their emotion to the stage, earning a standing ovation from all four of the coaches for their performance. And while Holly also impressed with her cover of The Carpenters' "Superstar," it was Girl Named Tom who Kelly picked to move on to the live shows.

"They sound amazing together, but then they showed how each of them sound alone, and they can hold their own!" Kelly raved.

However, in a heartwarming twist, Holly will also be moving on! The mother of two was chosen for a steal by both Ariana Grande and John Legend, and chose to join Team Ari for the upcoming live rounds.

"I have loved her from the start!" Ari raved of the singer, whom she originally block John for during the Blind Auditions. "Having her choose me was such a beautiful full-circle moment."

Last week, a source told ET that Ari has been loving her first season on The Voice so far.

"She has looked up to the coaches for a while and she and Kelly have really formed a strong friendship," the source noted. "Ariana loves that this allows people to see her talents beyond her singing -- she’s a writer and producer, and very knowledgeable about music. [She] is also having fun and loves joking around with the coaches."

As the Knockouts continue, check out ET's complete roundup of the season 21 team rosters to follow along with your favorites.

The Voice airs Mondays and Tuesdays at 8 p.m. PT/ET on NBC. See more in the video below.