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 to pull off such a shocker after 121st-ranked Mirjana Lucic-Baroni beat No 2-seeded Simona Halep on Friday.

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.

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, 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 past two US Open finals, says she had never seen Krunic play before.

Novak Djokovic says his fellow Serb Krunic bumped him out of a private physical therapy room in the players' gym before her second-round match earlier this week and ran into him in the gym again before her upset of Kvitova. "I hope she kicks me out every single day so she keeps on winning, if that's the lucky charm," he said with a laugh.

There was no upset for Williams, who easily moved into the fourth round by beating 52nd-ranked American Varvara Lepchenko 6-3, 6-3.

Williams, a 17-time Grand Slam singles champion bidding to win her third US Open in a row, 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.

I've been a casualty this whole year at Grand Slams. I'm just hoping to keep staying in there," said Williams who has lost early in every Grand Slam tournament since winning at Flushing Meadows last year.

Among the other major women's seeds to survive were No 11 Flavia Pennetta and No 17 Ekaterina Makarova.

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.

The top-seeded 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 22nd 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.

Third-seeded Stan Wawrinka, the Australian Open champion, got a welcome walkover into the fourth-round when his opponent, Blaz Kavcic, withdrew just before their scheduled match with a right foot injury.

Eighth-seeded Andy Murray needed four sets to get past Andrey Kuznetsov 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.

That sets up an intriguing fourth-round matchup with No 9 Jo-Wilfried Tsonga, who overcame tough winds in the Grandstand to beat Pablo Carreno Busta 6-4, 6-4, 6-4. His prediction: "I will come on the court as a fighter."

Tenth-seeded Kei Nishikori advanced in straight sets against No 23 Leonardo Mayer, and fifth-seeded Milos Raonic took three straight tiebreakers over Victor Estrella Burgos, who at 34 was the oldest man left in the draw.

In other key matches Saturday, American John Isner, seeded 13th, sought to defeat No 22 Philipp Kohlschreiber, the man who has beaten him in the third round the last two years.

Bouchard and 19-year-old Nick Kyrgios, a lanky Australian with a big serve who stunned Rafael Nadal in the fourth round at Wimbledon, are featured in the main night matches in Ashe.