NEW YORK -- Wimbledon champion Petra Kvitova was stunned in the third round of the US Open by a scrambling, moon-balling qualifier ranked No. 145.
Aleksandra Krunic, a 21-year-old Serb who until this week had not taken even a set from a player in the top 30, crumpled to the court in celebration and shock Saturday when she pulled off the 6-4, 6-4 upset of the third-seeded Czech.
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.
Bidding for a third consecutive US Open title, Williams moved into the fourth round by beating 52nd-ranked Varvara Lepchenko 6-3, 6-3.
Williams has faced American opponents in all three of her matches this week while extending her overall winning streak at Flushing Meadows to 17.
Williams had some trouble dealing with the whipping wind in Arthur Ashe Stadium but eventually took control against Lepchenko, a 2012 Olympic teammate and occasional practice partner.
Williams, a 17-time Grand Slam singles champion, is hoping to avoid her first season since 2006 without reaching at least one major final. She lost in the fourth round at the Australian Open, second round at the French Open, and third round at Wimbledon.
Kvitova, who would have ascended to the No. 1 ranking if she won the US Open and Williams had lost before the quarterfinal, committed 34 unforced errors against the fast-moving Krunic, who slid and sometimes went into a 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 that put Krunic a point away 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, a two-time Wimbledon champion who has never advanced past the fourth round at the US 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 US Open final the last two years, says she had never seen Krunic play before.
The Associated Press contributed to this report.