When it was all over, after the last of Kvitova's 34 unforced errors sailed wide, Krunic crumpled to the ground in disbelief at what she had just done - a qualifier ranked 145th, she toppled the No. 3 seed 6-4, 6-4, the latest departure by a top woman in the U.S. Open's first week.
''Of course, I didn't expect to win,'' said Krunic, a 21-year-old Serb who until this tournament had not taken even a set from a player in the top 30.
Even when Krunic took the lead in the second set and the Louis Armstrong Stadium crowd was cheering her on, ''I just told to myself, 'It's still on her; you still have nothing to lose.'''
''I really focused my 100 percent not to think about anything that is happening - about the court, about so many people, about Petra,'' she said.
Krunic's third-round victory Saturday was the second big shocker by a qualifier after 121st-ranked Mirjana Lucic-Baroni beat No. 2-seeded Simona Halep on Friday. Only three of the top eight seeds remain, No. 1 Serena Williams, No. 5 Maria Sharapova and No. 7 Eugenie Bouchard.
For Kvitova, who won her second Wimbledon title last month, the loss was huge. The big-hitting Czech would have ascended to the No. 1 ranking if she won the U.S. Open and Williams had lost before the quarterfinals.
Kvitova's hopes died in a rain of moonballs that bounced slow and high around her shoulders, out of her normal strike zone. And Kvitova didn't have an answer for the fast-moving Krunic, who slid and sometimes went into a gymnastic split to chase down every ball and make her opponent hit one more shot.
Kvitova too often failed to do so, including what should have been an easy putaway of a drop shot. She instead put it in the net, leaving Krunic a point from winning the first set. Kvitova sprayed a backhand long on the next point to lose it.
Those problems continued in the second set, and in an epic 27-stroke rally on match point, Krunic looped several towering shots before Kvitova sailed yet another forehand out to end it.
''She played really unbelievable tennis and she put a lot of balls back ... almost all of them,'' said Kvitova, who has never advanced past the fourth round at the U.S. Open. ''I was trying to fight and fighting every point, but it was so difficult. It wasn't really my day.''
Krunic next faces 16th-seeded Victoria Azarenka, a straight-sets winner over Elena Vesnina. Azarenka, who has lost to Williams in the past two U.S. Open finals, says she had never seen Krunic play before.
Novak Djokovic, the top seed on the men's side, says fellow Serb Krunic bumped him out of a private physical therapy room in the players' gym before her second-round match earlier this week. He said they ran into each other again in the gym Saturday before her upset of Kvitova.
Said Djokovic: ''I hope she kicks me out every single day so she keeps on winning, if that's the lucky charm.''