US Open: Petra Kvitova in shock loss to Aleksandra Krunic

Petra Kvitova has never been beyond the fourth round at Flushing Meadows.

Wimbledon champion Petra Kvitova became the latest shock exit in the women's draw at the US Open with a third-round loss to qualifier Aleksandra Krunic.

The 21-year-old Serb, ranked 145th, beat Czech third seed Kvitova in straight sets, 6-4 6-4.

The 24-year-old joins Simona Halep, Agnieszka Radwanska, Angelique Kerber and Ana Ivanovic in falling early.

Top seeds Novak Djokovic of Serbia and American Serena Williams won in straight sets to reach round four.

Djokovic, 27, beat America's Sam Querrey 6-3 6-2 6-2, while Williams, 32, beat compatriot Varvara Lepchenko 6-3 6-3.

Djokovic, who won the US Open in 2011, will play Germany's Philipp Kohlschreiber next.

Kohlschreiber beat the last American remaining in the men's draw, John Isner, 7-6 (7-4) 4-6 7-6 (7-2) 7-6 (7-4). It was the third time in a row the German has knocked Isner out of the US Open.

Djokovic is joined in the fourth round by Britain's number one Andy Murray, who beat Russia's Andrey Kuznetsov 6-1 7-5 4-6 6-2.

Murray will play France's ninth seed Jo-Wilfried Tsonga for a place in the quarter-final after the Frenchman beat Spain's Pablo Carreno Busta in straight sets.

Australian Open champion Stan Wawrinka reached the US Open fourth round by walkover when Slovenian Blaz Kavcic withdrew from their third-round match with a right foot injury.

With Kvitova's departure, there are now only five of the top 10 women's seeds left in the tournament.

"I'm very disappointed," said Kvitova. "I wanted to win today, and unfortunately I didn't.

"She played really unbelievable tennis and she put a lot of balls back. Almost all of them. For me it was very difficult just to play only on the winners."

The Czech, who had her right thigh heavily strapped, looked as if she might turn things around when she came from 4-2 down to level at 4-4 in the second set, but Krunic showed composure to seal the victory.

The economics student, who had never taken a set off a top-30 player and never won a Grand Slam match before this tournament, faces 16th seed Victoria Azarenka of Belarus next.

Arthur Ashe stadium

Flavia Pennetta (Ita) (11) beat Nicole Gibbs (US) 6-4 6-0

Novak Djokovic (Ser) beat Sam Querrey (US) 6-3 6-2 6-2

Novak Djokovic has a 8-1 head-to-head record against Sam Querrey

"My game is peaking at the right time, at the right tournament. This is where I want to play my best tennis," said Djokovic.

"I haven't done as well as I wanted in lead-up tournaments, Toronto and Cincinnati. Obviously emotionally I wasn't ready for those tournaments. But the upside of me losing early in Cincinnati is that I had more time to prepare for US Open, last Grand Slam of the year."

Serena Williams (US) (1) beat Varvara Lepchenko (US) 6-3 6-3

Eugenie Bouchard (Can) (7) beat Barbora Zahlavova-Strycova (CZE) (30) 6-2 6-7 (2-7) 6-4

"I did way more unforced errors (39) than I expected," said Bouchard, who lost to Kvitova in the Wimbledon final. "To just give away so many free points is really frustrating.

"I realised in the third I can still stay with her, I don't have to go for crazy shots all the time. I was a bit too impatient in the second set."

Tommy Robredo (Spa) beat Nick Kyrgios (Aus) 3-6 6-3 7-6 (7-4) 6-3

Belinda Bencic (Sui) beat Jelena Jankovic (Ser) (9) 7-6 (8-6) 6-3

"I'm so tired," said Bencic, the youngest female Grand Slam quarter-finalist since 2006 at the age of 17. "It was always a dream to play on this court. I was overwhelmed, everything was so huge."

Louis Armstrong Stadium

Aleksandra Krunic (Ser) beat Petra Kvitova (CZE) (3) 6-4 6-4

"It was an honour for me to be on the same court as Petra, who is a great champion," Krunic told the Louis Armstrong Stadium crowd after the 98-minute match. "I watched both of her finals matches at Wimbledon.

"Going to play against such a champion meant to stay on the court as long as possible. Of course, I didn't expect to win. Of course, I hoped to. I'm very happy."

Andy Murray (GB) (8) beat Andrey Kuznetsov (Rus) 6-1 7-5 4-6 6-2

"I got off to a good start, which obviously helped," said Murray. "I had a bit of a cushion when he started to get back into the match.

"He played some good stuff at the end of the second and all the way through the third. I'm happy I stayed solid. It was tough."

Philipp Kohlschreiber (Ger) (22) beat John Isner (US) (13) 7-6 (7-4) 4-6 7-6 (7-2) 7-6 (7-4)

Stan Wawrinka (Sui) (3) beat Blaz Kavcic (Slo) walkover

Tomas Berdych (Cze) (6) beat Teymuraz Gabashvili (Rus) 6-3 6-2 6-4

"The conditions were really tough," said Berdych. "I would say one of the wettest days you can have on court, which is something that's not very pleasant."

Gael Monfils (Fra) (20) beat Richard Gasquet (Fra) (12) 6-4 6-2 6-2

"I feel good. I've had a great summer. I think I have tough matches. I think I had some opportunity against the big guys and I didn't make it, so I won't say great. I'll say OK."

Grandstand

Milos Raonic (Can) (5) beat Victor Estrella Burgos (Dom) 7-6 (7-5) 7-6 (7-5) 7-6 (7-3)

"It's good to get the win," said Raonic, who plays Japan's Kei Nishikori in the fourth round.

"It's good to put myself in this position in the fourth round to go further in this tournament and give myself an opportunity to play better. I believe I can play better. I believe I will."

Victoria Azarenka (Blr) (16) beat Elena Vesnina (Rus) 6-1 6-1

Victora Azarenka plays Serbia's Aleksandra Krunic in the fourth round

"I had an annoyed look on my face? No, I'm just fierce like that," said Azarenka. "It's my way of feeling good, what I show on my face.

"I don't really have a mirror right there, but my emotions are really positive and all I talk about to myself is positive things."

Jo-Wilfried Tsonga (Fra) (9) beat Pablo Carreno Busta (Spa) 6-4 6-4 6-4

"I did a good performance today," said Tsonga. "Happy to go through, of course. I'm playing good. I just expect to continue in this way."

Grigor Dimitrov (Bul) (7) beat David Goffin (Bel) 0-6 6-3 6-4 6-1

"He played a great, great first set," said Dimitrov. "Whatever I was trying to do was just not enough. His game was just too good in the first set. But I didn't let go of my belief, especially coming into that second set. I knew eventually I was playing my game."

Quotes of the day

Milos Raonic on hitting a ball boy with a 143mph serve: "I didn't know I hit him in a very sensitive spot because the guy stood up pretty well. I'm not sure if I could if something hit me there at 143 miles an hour."

"Really? Where is my cake, then? Like for real. Tonight? Thank you. Put some icing on it." Victoria Azarenka is told she has racked up 100 Grand Slam wins.

Is Novak Djokovic better in the wind than other players? "He's better in any condition than other players," says Sam Querrey. Was fatigue a factor? "I was only out there for about an hour and 10 minutes, so not really," the American adds.

"I will sing an original song I wrote called African Sun. I wrote this song to give hope to the children in Africa." Nine-year-old Zia Uehling is invited by Novak Djokovic to sing during his press conference.

"I can't believe I'm in the second week. It's like a dream come true for me at this point," says Serena WIlliams to a stunned media. "I'm being sarcastic," she adds.

Gael Monfils does not plan a wild birthday celebration on Monday. "I think I'm just going to call my mom. I'm born like earlier than that. I think it's already my birthday. I was born early in France. I just call my mom. That's more important."