Serving for the match at 5-4 in the second set, the 21-year-old world number 145 caught Kvitova flat-footed with a perfectly timed drop shot to take a 40-0 lead.
"I was like, 'Oh, my God, I have triple match point! Maybe I can really win it,'" Krunic said, adding that she was still trying to absorb the fact of her second-round victory over American Madison Keys.
"It's like so many things happening in my mind," she said. "I'm trying to believe it, but when I believe it I'm afraid that I'm going to have so much emotions. So I'm trying to keep it all together.
"My coach is already screaming and jumping, and I'm trying to stay away from him so I don't listen," she said, holding up her hands as if to ward off the excitement.
The pint-sized dynamo made the most of her 5-foot-4-inch (1.63m) frame, making up for her lack of power with sheer determination.
"I didn't really expect how she played so well," admitted Kvitova, whose second Wimbledon title in July was followed by a couple of lackluster results before she lifted the trophy in New Haven on the eve of the Open.
"Suddenly she just pushed very hard the ball. It was just coming so quickly then, so it was really difficult. I just didn't really to have an answer for it."
Krunic's game plan, to extend the rallies and try to push Kvitova into mistakes, worked to perfection. The 24-year-old Czech committed 34 unforced errors and surrendered her serve five times.
But Krunic, whose second-round win over Keys was her first against a top-30 player, seemed astonished that she'd pulled it off against a player she was pulling for in two Wimbledon finals.
"I like Petra a lot as a person," Krunic said. "She's very down-to-earth and I respect her a lot. I like when I can say hi and talk to the players that are much higher ranked than me and I don't feel such a difference in our levels. Because still we're all human beings."
Krunic, whose career path has been smoothed by financial support from a family friend, knows she's lucky in that regard, as well as in having a wealth of talented fellow Serbians to lean on, including world number one Novak Djokovic, and her Fed Cup doubles partner Jelena Jankovic.
Djokovic might even be part of a new pre-match ritual.
"You know, actually I kicked out Djoko from the quiet room in the gym before my match with Madison, and today before my match I also saw him in the gym," she said.
"We were kind of joking after the match, he said 'Keep kicking me out if you're playing good.'"