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.
‘‘Of course, I didn’t expect to win,’’ said Krunic, the second qualifier in two days to pull off such a shocker after 121st-ranked Mirjana Lucic-Baroni beat second-seeded Simona Halep.
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,’’ she said.
‘‘I really focused my 100 percent not to think about anything that is happening — about the court, about so many people, about Petra.”
After a week of upsets on the women’s side, only three of the top eight seeds remain, No. 1 Serena Williams, No. 5 Maria Sharapova, and No. 7 Eugenie Bouchard.
Kvitova committed 34 unforced errors against the fast-moving Krunic, who slid and sometimes went into a split to chase down balls 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 past two US Open finals, says she had never seen Krunic play before.
There was no upset for Williams, who easily moved into the fourth round by beating 52d-ranked American Varvara Lepchenko, 6-3, 6-3.
Williams, a 17-time Grand Slam singles champion, 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.
The No. 1-ranked and No. 1-seeded Williams has faced American opponents in all three of her matches this week while extending her overall winning streak at Flushing Meadows to 17. She is trying 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.
Bouchard, the Wimbledon runner-up, pulled out a suddenly tight match Saturday night, edging 30th-seeded Barbora Zahlavova Strycova, 6-2, 6-7 (2-7), 6-4, to reach the fourth round for the first time.
Bouchard was the only woman to make it to at least the semifinals at each of the year’s first three Grand Slam tournaments. At Wimbledon, she became the first Canadian to appear in a major singles final.
To get to the quarterfinals at Flushing Meadows, Bouchard will need to beat No. 17 Ekaterina Makarova.
Zahlavova Strycova fell to 1-28 in matches against opponents ranked in the top 10.
The men’s side, meanwhile, remained largely true to form. None of the top 10 and only two of the top 20 seeded men had lost entering Saturday.
Top-seeded Novak Djokovic continued that trend with a routine 6-3, 6-2, 6-2 victory over American Sam Querrey, earning a spot in the Round of 16 at a 22d consecutive Grand Slam tournament. Djokovic counts the 2011 US Open among his seven major titles and has played in the past four finals at Flushing Meadows.
Djokovic needed all of 15 minutes to take the first five games against Querrey. He had no trouble handling the big serves of Querrey, breaking the former top-20 player seven times in the 1-hour-26-minute match.
Eighth-seeded Andy Murray needed four sets to get past Andrey Kuznetsov, 6-1, 7-5, 4-6, 6-2, in humid, overcast conditions, his first day match since he battled cramps over much of his body in a first-round victory over Robin Haase.
‘‘I ate properly, I drank properly, I felt a lot better today,’’ he said.
Tenth-seeded Kei Nishikori advanced in straight sets against No. 23 Leonardo Mayer, 6-4, 6-2, 6-3.