The relatively unknown name on the LPGA tour of Ewart was the talk of the clubhouse after the two-times English Amateur Champion stunned Pettersen 3 and 1 at the Hamilton Farm Golf Club in New Jersey.
Ewart, a former University of New Mexico player, made light of her 202nd ranking as she came from two holes down for her victory.
"After number two I was like, I am going to get absolutely thrashed," Ewart said.
"On paper, I'm not supposed to win at all, so there's no pressure on me.
"I just went out there and played fearless golf because I was attacking the pins."
It was not the only shock result on the opening day of play, with Paula Creamer, Brittany Lincicome and Michelle Wie also losing to relatively obscure players.
Creamer was beaten 2 and 1 by Jennifer Johnson, Linicome fell by the same scoreline to Ryann O'Toole and Wie was sent packing 3 and 2 by Mina Harigae.
Fourth-ranked Ai Miyazato was also upset by Mariajo Uribe, the Colombian 2-up after winning four of the last five holes.
And world number one Yani Tseng, number two Na Yeon Choi and number five Cristie Kerr only scraped through their match-ups.
"It's not an easy course to play," said Tseng, who escaped with a 1-up against Jeong Jang. "This is the top 64; that's why we play here. So you cannot expect the first one is going to be easy to win."