In a pre-taped segment before he came out to compete, Ketterer was brought into a darkened room with a video projected against the wall. After sitting down and pressing play, he was treated to a video montage of messages from friends, family, and others who love him.
Ketterer works as a pediatric nurse who specializes in juvenile mental health issues, and some of the parents of the children he's helped thanked him for saving their kids and changing their lives.
From a couple who relied on Ketterer for emotional support when they lost their daughter several years ago to a man who was inspired to want to be a father by a supportive letter the singer wrote to him in the hospital, the touching montage featured dozens of tearful videos.
Ketterer -- who is the biological father of a teenage daughter and the adoptive father of five young sons -- was already getting choked up by the messages from old friends and former patients. However, when his kids shared their own words of support, he couldn't hold back the waterworks.
"You're just the best father. And I love you so much," Ketterer's adoring sons said sweetly, as an overwhelmed Ketterer wiped tears from his eyes.
Taking to the stage after the pre-taped introduction, Ketterer delivered an emotional, raw rendition of the classic hit "Ain't No Mountain High Enough," and while his nerves about being in the finals were somewhat apparent, he pulled off a powerhouse performance that earned a standing ovation from the crowd.
"I would love to be able to give your family the million dollars, because you're just an amazing guy," Cowell said, stuttering at first in what felt like an effort to keep from crying. "So many people have spoken about you. You were nervous [tonight], but that's what makes you human… you're a great, great person, Michael."
For more on Ketterer's amazing AGT journey -- including the heartfelt performance that made Cowell tear up during the semifinals, check out the video below.
Tune in on Wednesday to see if Ketterer winds up the winner of this season when the America's Got Talent finale results show kicks off at 5 p.m. ET/PT on NBC.