AGT 's Sara James Is 'So Grateful' After Simon Cowell Calls Her Performance the 'Best Closure of the Show'

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The America's Got Talent judges feel confident a teen semifinalist from Poland will be the show's next big star.

During Tuesday night's live show, 14-year-old Sara James wowed the panel with a moody, slower version of the beloved Elton John classic "Rocket Man," with Howie Mandel going so far as to call it a "winning performance."

Once backstage, James told PEOPLE how shocked she was to see all four judges on their feet at the end of her performance.

"It's crazy, it's mad," she said. "I'm just a teenager from Poland, I don't even know what I'm doing here, do you know what I mean? Thank you so much to all the judges. I'm so grateful."

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The singer first made waves during her audition when her stunning rendition of Billie Eilish's "Lovely" compelled Simon Cowell to press his coveted Golden Buzzer.

James was quick to show her appreciation for the hard-to-impress judge on Tuesday, thanking him for "giving me the opportunity to stay on this stage."

Cowell, 62, admitted that James' show-closing performance was "probably the best closure of the show we've had all season." During the live show, he referred to the song choice as "genius" and his compliments continued backstage.

"That was one of those moments you don't see very often on any show — it was incredible," he told PEOPLE. "I've done this a long time and there are those occasions whether it's Carrie Underwood, Kelly Clarkson, Fantasia, Leona Lewis where someone has just got everything right on the day it matters."

"I think we witnessed one of those moments tonight," he added, noting that he also spoke to James' father earlier in the night. "He's so happy and I really am happy for her."

Trae Patton/NBC/Getty

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The other judges were also quick to sing her praises, with Mandel, 66, calling her "a star" after the show. "She looked beautiful, sounded beautiful, and just performed with such grace as if she's been doing it for decades," he said.

Heidi Klum also pointed out that "she sounds like a pro... it was yummy."

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James told PEOPLE she was determined to "do something different" for her first live show appearance and wanted to take on "a real legend" like John, 75.

"I didn't know his music that well and then I watched the movie Rocketman," she explained of learning his big hits. "It was so amazing and inspired me to do the song tonight."

All eyes are on the vocalist since her audition caught the attention of Eilish, 20, who retweeted the performance after it aired. "I still can't believe it, to this day," James gushed. "I looked at it and ran to my mom, who was in the bathroom, and I yelled, 'Mom, look at that, it's Billie.' It's so crazy."

Trae Patton/NBC Sara James on America's Got Talent

The support of James' mom has also helped carry her throughout her AGT experience and she was thrilled to have her in the audience on Tuesday.

"My mommy has always been my number one supporter, my number one fan," she said. "She has always taken care of me even when I'm not the easiest child. She has my back, does the thing, and I love her so much."

As she awaits the results of the public vote, the teen is still in awe of how "unreal" it's been to see the outpouring of online support since her first appearance on the AGT stage.

"I couldn't ever believe that I could be here," she explained. "I'm from a village with 3,000 people, five shops and two gas stations, and I'm here now! It's crazy!"

Fans will find out if James is one of two viewer-voted contestants to move onto the season 17 finale when the AGT results show airs Wednesday at 8 p.m. ET on NBC.