Before this summer, Riske had fallen at the first hurdle on all five of her Grand Slam appearances, but she broke her duck with a third round run at Wimbledon and has now gone one better in New York.
The American's performances over the past few months have seen her rise nearly 100 places in the rankings, reaching a career-high No.81 on Monday following a second round appearance in New Haven.
Her conqueror on that occasion was none other than Kvitova, who edged her out in three tight sets. However, Riske was leaving nothing to chance out on Court 17, an impeccable serving display powering her to a 63 60 victory.
"I've got a new confidence in myself," a tearful Riske said in her on-court interview. "I believe that I belong here.
"It was really emotional for me. But it's cool because I feel like I'm playing within myself and not doing anything too crazy. It's just really comfortable for me out there."
Despite the disparity in their respective rankings, Riske traded on equal turns from the start, pinning Kvitova to the baseline. Unable to dictate the exchanges, the Czech's frustration was clear for all to see, particularly after gift-wrapping the first break with a double fault in the fifth game.
The set soon followed and when she won a titanic game at the start of the second, Kvitova's self-belief visibly began to evaporate. Kvitova could do nothing to wrestle back the momentum and Riske was soon at match point, closing it out with a crisp backhand winner before raising her arms in delight.
Four years ago, Riske was all set to play to accept a tennis scholarship to Vanderbilt University, only to make an 11th hour U-turn when a family friend offered to sponsor her in the professional ranks.
However, despite her travails on tour, Riske never regretted her decision. "It's really tough out here. Every week isn't like this," she said. "But that's the great part about it, too, because if you do have a bad week you can kind of follow it up the following week with something better.
"There were definitely times of doubt. My team was so supportive and pushed me through and I'm definitely in a better place now."
Riske's friend and fellow American Christina McHale narrowly failed to join her in the last 16 after being edged out by No.13 seed Ana Ivanovic. McHale, who entered the tournament with just one win since May, failed to serve out the match in the second set before losing, 46 75 64.
"Well, I tried not to think of the score at the end of the second," Ivanovic said. "I just tried to keep my head down and be aggressive. I took some risks and it paid off so I'm very pleased.
"I was really passive in the beginning today and didn't move my feet around the ball. But at the end of the second set and the beginning of the third I began to serve a bit better. I stayed calm on my serve and it worked really well, which I'm pleased about because she's a great player and played really well."