NEW YORK—Petra Kvitova sailed another forehand, slumped her head, and leaned on the handle of her Wilson racquet like a weary traveler hanging onto a cane during a draining journey.

On the opposite side of the net, Serbian qualifier Aleksandra Krunic bounced up and down in eager anticipation. Even when the 145th-ranked Krunic wasn't playing points, she looked like a woman going places. Mixing her spins masterfully and sliding around the court in eye-popping retrievals, Krunic ran Kvitova right out of the U.S. Open in a 6-4, 6-4, third-round stunner.

A day after Mirjana Lucic-Baroni used her stinging forehand to upset second-seeded Simona Halep, Krunic relied her court coverage, guile, and some gutsy strikes to send the third-seeded Czech packing.

The two-time Wimbledon champion took the court with as many major championships as Krunic had Grand Slam victories. Fresh off her 13th-career title in New Haven a week ago, Kvitova permitted just seven games through the first two rounds and finally looked ready to go deep in the only Grand Slam event where she's yet to reach the final four. None of that mattered much to the Moscow-born Krunic, who exhibited the flexibility of a gymnast sliding into splits to flick back shots. Initially, Kvitova looked a bit taken aback by those scrambling gets, but by the end there was a weary resignation in her eyes.

Though she stands nearly eight inches shorter and faced an immense power disparity, Krunic, who cites the drop shot as her favorite shot, dragged Kvitova out of her comfort zone and into obscure areas. Wisely taking the pace off, Krunic looped some heavy topspin forehands, short slice backhands, and used the dropper shrewdly, creating misery for Kvitova. The 21-year-old played a much cleaner match, committing 20 fewer unforced errors. And despite the fact Kvitova more than doubled her opponent's winner output in the opening set (17 to eight), Krunic converted both of her break points in snatching the 41-minute opener.

The fact Krunic had beaten another big-serving power player, 27th-seeded American Madison Keys, in round two seemed to help prepare her for the aggressive baseliner's explosive pace. Krunic broke for a 3-2 second-set lead, consolidating at 15 for 4-2.

Some fans shouted "come on Petra!" trying to implore the Wimbledon champion to come back. Playing with taping around her right thigh, Kvitova tried to shorten points by attacking the net. She saved a pair of break points, pounding a serve down the middle to hold for 3-4 after 82 minutes of play. When Krunic narrowly missed a running pass down the line, Kvitova broke back for 4-all with a shout, showing her opponent she was ready to fight.

One jaw-dropping get altered the entire set. At 30-all, Kvitova had Krunic on a string, belting a sharp-angled backhand to open the court. So Krunic, sneakers squealing, made a Nole-esque sliding retrieval to force one more shot. With an open expanse of court down the line, a tight Kvitova gagged and sailed a forehand long. Though she saved the first break point, Kvitova launched a backhand long on the second, gifting the break and a 5-4 lead.

Tormenting the prohibitive favorite with another backhand drop shot winner to reach triple match point, Krunic closed on Kvitova's 33rd error, dropping flat on her back, flipping her racquet aside and staring up at the sky in exhilaration as the crowd erupted. Krunic's exuberant reaction—and candor—was refreshing.

"Of course, I didn't expect to win," she told CBS' Mary Joe Fernandez afterward. "Of course, I was hoping to win at least a set. I managed to win the match somehow, I don't know how."

Ten minutes after the match, Krunic was still signing autographs with a wide smile, savoring the moment and the interaction with fans until a USTA staffer approached her and said "we've got to get the court ready for the next match now." Something tells me the high-energy Krunic is already getting ready for her fourth-round meeting with either 16th-seeded Victoria Azarenka or Elena Vesnina.