The 19-year-old captured the decisive break at 5-5 in the second set and served out the match nervelessly under the lights of Rod Laver Arena, stunning the two-time Wimbledon champion.
"I knew I was going to have to play my best to beat her," said Keys, who is coached by former world number one Lindsay Davenport. "I just tried to stay solid and consistently keep doing what I was trying to do.
"This is my first time in the second week of a grand slam.
"I feel good about it. I feel like I've made a lot of improvements ... but at the same time it's one of those things where I want more. I want to keep doing better.
"So I'm not really getting ahead of myself and letting myself get too excited over this. I'm just treating it like another win."
Keys' win made it four American women in the last 16 at Melbourne Park, the most since five made it to the fourth round of the 2003 Australian Open.
"There's four women into the fourth round. That's big. I think it's (U.S. tennis) doing pretty well," Keys said. "It's going in the right direction."
Keys' hitting against Kvitova justified her reputation as one of the most powerful ball strikers in the game, something that was proved at last year's French Open when officials measured the average speed of the groundstrokes of every player.
The teenager's average speed was faster than anyone else, men included, and she used that power to pressure Kvitova at the baseline on Saturday.
"She was going forward for it, playing real aggressive from the first point. I think that she played really, really fast, aggressively," said Kvitova.
"She didn't make a lot of mistakes at all (and) I felt the pressure ... from the baseline. I was staying on the line and try to hit it back, but it didn't help. It was tough."
Keys will now play compatriot Madison Brengle for a place in the quarter-finals, with the 24-year-old Brengle, a fan of the Lord of the Rings films, earlier joking that it could be a fight for the survival of the name.
"A couple weeks ago she sent me one of those funny (Internet) memes," Brengle told reporters.
"It was a picture of like one kid holding a door over another kid. We need to recreate this picture. There can only be one."
(Reporting by Ian Ransom and Greg Stutchbury; Editing by Alan Baldwin)
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