Pegula was trailing 2-0 in the second set before she snatched the momentum from her Czech opponent to win 1-6, 6-4, 6-2 and qualify for her first major final.
"I think the crowd really helped me get some adrenaline into me... I think I was just very, very flat. I wasn't even nervous," Pegula told reporters.
"At the end of the first she was playing some extremely high level so I just had no rhythm... holding that game, I was able to just find some adrenaline and get my legs under me. You know, just try and chase down every single ball that I could."
The 30-year-old said she was more nervous before facing world number one Iga Swiatek in the quarter-finals, a match she won in straight sets.
"Today I was just, like, whatever. Maybe that was bad, because I clearly came out super flat," Pegula joked. "I was trying to think that maybe it was good that I felt really loose, but clearly I was a little too loose."
Pegula, who plays second seed Aryna Sabalenka in the final on Saturday, said she knew a way to frustrate the Australian Open champion who beat her in last month's Cincinnati Open final.
"Cincinnati, she served unbelievable, and I felt like I still had chances in that match... I have to get her moving, serve smart, and try and put some pressure on her serve," she said.
New York-born Pegula said she had fulfilled a childhood dream by reaching the US Open final, and winning the title would mean the world to her.
"If you would have told me at the beginning of the year I'd be in the finals of the US Open, I would have laughed so hard," she said.
"To be able to overcome all those challenges and say that I get a chance at the title Saturday is what we play for as players, let alone being able to do that in my home country here, in my home slam. It's perfect, really."
