"My heart is just racing so hard right now. I just saw Carrie in the hallway, actually, and I don't know what to say," Rhett told ET's Rachel Smith backstage after host Keith Urban announced the pair's big wins on Wednesday night. "Good grief! Carrie's an idol of mine, and just to tie this with her, it's unbelievable."
The "Die a Happy Man" singer said he's been working hard on winning the prestigious award "for so long," and is "so grateful" to have earned it this year. Sharing it with Underwood is a plus.
"It feels absolutely amazing, because Carrie has set the bar to me for entertainment for so long, vocalist, songwriter, amazing artist. And so just to be nominated in this category is a huge deal, but to actually tie it with her for the first time in ACM history, I'm honestly speechless, to tell you the truth... I don't have any words. I'm sorry, my heart is beating so fast," he said.
"It really does take an army of people... this is absolutely mind-blowing and will never be taken for granted.," Rhett added. "And I'm so glad that we got to do this and even amid the crazy times."
With Wednesday's trophy, Underwood became the first woman to win the Academy of Country Music Award for Entertainer of the Year three times. She previously won Entertainer of the Year in 2008 and 2009. Taylor Swift, with wins in 2010 and 2011, is the only other woman who's won the award multiple times. Garth Brooks holds the record for most Entertainer of the Year wins overall, with six total.
"It feels great. Obviously there's so many people that make our worlds go round, so just to be able to hold this for myself, of course, but also for them, it means a lot. And I like to share," Underwood told ET with a laugh. "I like to share with people, so I just think it's really great that ... it's 2020, nothing's going to be normal!"
"It's definitely the year of.. nobody knows what's going to happen," she continued. "Nobody knows. It's the most unpredictable year."
Rooting for Underwood to win were many of her peers -- including Maren Morris and Mickey Guyton.
"I don't even know how many years it's been since there was an Entertainer [of the Year] that's been a woman at the ACMs, so I feel like she has my vote, and it's not because she's a woman, it's because she ups her game with every single album cycle, ever tour," Morris told ET. "She changes production every time, she always raises the ceiling for the rest of us, and I feel like she deserves to walk away with it. So, she's got my vote."
"Women are so important, our voices matter, we matter, we are essential to life, and sometimes it doesn't always feel like that. And if Carrie Underwood wins this award, that means we all win in country music. All women win in country music," Guyton added. "It can be hard sometimes, and we need this win, because it gives us hope. She is our hope right now. And it would just mean the world to me, especially."
During Wednesday's awards ceremony, Underwood also delivered a powerful tribute to trailblazing female Opry members by performing a medley of songs by Patsy Cline, Loretta Lynn, Barbara Mandrell, Martina McBride, Reba McEntire and Dolly Parton.
"It was my honor. Those are huge shoes to fill. If they had let me sing songs to all the women I idolize, we would have been there for a really long time. But it's just nice to be able to -- they're the girls I grew up listening to, and they taught me how to sing. And to be able to honor them tonight was just really special for me," Underwood gushed.
See more on the ACM Awards in the video below.