NEW YORK – Instead of trying to match Wimbledon champion Petra Kvitova’s power, Aleksandra Krunic confounded her with looping shots and frustrated her by chasing down every ball.

When it was over, after the last of Kvitova’s 34 unforced errors sailed wide, Krunic dropped to the ground in disbelief at what she had 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.

“Of course, I didn’t expect to win,” said Krunic, a 21-year-old from Serbia.

After three rounds, three of the top eight seeds remain, No. 1 Serena Williams, No. 5 Maria Sharapova and No. 7 Eugenie Bouchard.

Williams defeated Varvara Lepchenko 6-3, 6-3 in a matchup of Americans, and Canadian Bouchard beat No. 30 Barbora Zahlavova Strycova of the Czech Republic 6-2, 6-7 (2-7), 6-4.

Czech 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 seemingly chase down every ball and make her opponent hit one more shot.

“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.

As for the men, once again, American John Isner’s quest ended in the third round. Once again, it happened with a loss to Germany’s Philipp Kohlschreiber at that stage.

And once again, there are zero U.S. men in the round of 16 — something that had never happened until it did last year at the country’s tennis championship, which was first played in 1881.

No. 22 Kohlschreiber defeated the 13th-seeded Isner 7-6 (7-4), 4-6, 7-6 (7-2), 7-6 (7-4).

“It’s disappointing for me personally — not for America as a whole,” the 6-foot-10 Isner said.

Top-seeded Novak Djokovic of Serbia cruised past American Sam Querrey 6-3, 6-2, 6-2.