Fernandez, who celebrated her 19th birthday on Monday, shocked No.2 seed Aryna Sabalenka 7-6 (7-3) 4-6 6-4 in two hours, 20 minutes on Friday (AEST).
British teenager Emma Raducanu, 18, awaits in Sunday's (AEST) final at Flushing Meadows.
The decider will be the eighth grand slam final in the Open Era between teenagers and first since the 1999 US Open when Serena Williams defeated Martina Hingis. Williams, who remains an active player, has gone on to win 23 Major titles, while Hingis won five.
"Impossible is nothing. Like my dad would tell me all the time there's no limit to my potential to what I can do," Fernandez said post-match.
"Nothing's impossible. There's no limit to what I can do. I'm just glad that right now everything's going well."
Fernandez only claimed her maiden WTA Tour title in March, triumphing at the Monterrey Open, while she is only playing her third grand slam, never going further than the third round until this tournament.
The Montreal-born talent labelled her US Open run as "magical" having knocked out top-five trio Sabalenka, defending champion Naomi Osaka and No.5 seed Elina Svitolina, along with three-time slam winner Angelique Kerber.
"I think I've been doing some things incredible," Fernandez said. "One word that really stuck to me is 'magical' because not only is my run really good but also the way I'm playing right now.
"I'm just having fun, I'm trying to produce something for the crowd to enjoy. I'm glad that whatever I'm doing on court, the fans are loving it and I'm loving it, too. We'll say it's magical."
Fernandez also revealed when she was in grade six, a teacher had told her to stop playing tennis and focus on school.
"I'm just glad that she told me that because every day I have that phrase in my head saying that I'm going to keep going, I'm going to push through, and I'm going to prove to her everything that I've dreamed of I'm going to achieve them," she said. "I think now I can say that I've done a pretty good job in achieving my dreams."