The two-time Wimbledon champion was humbled 6-4 6-4 at Armstrong Stadium, as Krunic's fairytale run extended into the fourth round.
Kvitova, who landed 63 per cent of her first serves but won just 26 points when doing so, said she did not get enough free points and Krunic capitalised in the rallies.
"Yeah, my serve didn't work great today," the Czech third seed said.
"But anyway, I mean, I won some matches without my serve as well. It's always about me.
"Of course if I have a better serve maybe it's just helping me a little bit, but it was a lot of rallies and it was of course for her [advantage] these rallies."
Krunic, 21, had too much power for Kvitova, according to the third-round loser.
"I knew a little bit what she's playing, but I didn't really expect how she played so well. Really, I mean I was fine, and suddenly she just pushed very hard the ball," Kvitova said.
"It was just coming so quickly then, so it was really difficult, you know.
"She was trying to do everything, but of course it's great [for her]. I just didn't really to have an answer for it."
Kvitova said the women's tour was as closely fought as ever before, with only three top-eight seeds - Serena Williams, Maria Sharapova and Eugenie Bouchard - into the last 16.
"I think it's the women's tennis. I'm not pretty sure what's happening. This year for example, all the grand slams have a big upsets. It's same here," Kvitova said.
"I think just the tennis is so close. The levels are very close. Everybody really can beat everyone from the top, so it's really difficult to stay on the top.
"It's a lot of pressure over there. And the players are better and better, so it's really tough."