( CNN ) -- Day six of the U.S. Open at Flushing Meadows and the shocks just keep on coming in the women 's draw . Reigning Wimbledon champion Petra Kvitova is the latest casualty after crashing out in the third round to Aleksandra Krunic , a player ranked 145 in the world . The Serbian qualifier made light work of the heavy-hitting opponent from the Czech Republic winning in straight sets 6-4 6-4 in one hour 34 minutes . To say that Kvitova was off her game is an understatement as she lost serve five times and racked up 34 unforced errors . But rather than focus on the shortcomings in her own game on Saturday , Kvitova chose to praise her opponent . `` I think she played really unbelievable tennis , and she put a lot of balls back , '' Kvitova said . `` I did mistakes , and I was really trying everything that I could in that moment . I was trying to fight and fighting every point , but it was so difficult . It was n't really my day . She played really great tennis today . '' By her standards , Kvitova has a poor record at the U.S. Open having never got further than the fourth round . But the 24-year-old had reason to hope that this year might be different after claiming a second Wimbledon title in July and her win at the New Haven tournament last week . But her dreams have been extinguished for another 12 months as she becomes the fifth top 10 seed to depart the women 's tournament . Kvitova joins second seed Simona Halep , fourth seed Agnieszka Radwanska , sixth seed Angelique Kerber and Ana Ivanovic , the eight seed -- all of whom have already packed their bags . For Krunic , who will now play Belarusian Victoria Azarenka in the fourth round , it was a victory to savor . `` It was an honor for me to be in the same court with Petra . Of course , I did n't expect to win . Of course , I was hoping at least to win a set , but I managed to win a match somehow . '' Azarenka , seeded 16 at Flushing Meadows this year , eased through to the last 16 with a 6-1 6-1 win over Russia 's Elena Vesnina . Reigning champion Serena Williams is also safely through to the second week following a 6-3 6-3 win over Varvara Lepchenko while 11th-seed Flavia Pennetta from Italy will face Australia 's Casey Dellacqua for a place in the quarterfinals . Pennetta saw off Nicole Gibbs of the U.S. in straight sets 6-4 6-0 , while Dellacqua , the 29th seed , needed three sets to get past Karolina Pliskova , winning 6-3 3-6 6-4 in one hour 54 minutes . In the men 's competition , the top seeds are faring better . Champion of two years ago Andy Murray , seeded eighth this year , progressed to the fourth round with a four-set win over Russia 's Andrey Kuznetsov 6-1 7-5 4-6 6-2 . `` I got off to a good start , then had a bit of a cushion when he started to get back in the match , '' Murray said . `` I just tried to stay solid in the fourth set and thankfully it paid off . '' The 27-year-old Scot 's reward is a match up against Jo-Wilfried Tsonga . The French ninth seed beat Spain 's Pablo Carreno Busta in straight sets 6-4 6-4 6-4 . World No . 1 Novak Djokovic did n't break sweat against Sam Querrey , beating his American opponent 6-3 6-2 6-2 to set up last 16 tie against either Philipp Kohlschreiber or American 13th seed John Isner . Earlier on Saturday , Milos Raonic won in straight sets against Victor Estrella Burgos of the Dominican Republic . The number five seed from Canada will now play 10th seed Kei Nishikori in the last 16 . The Japanese No . 1 beat Argentina 's Leonardo Mayer 6-4 6-2 6-3 .
Reigning defending champion the latest top seed to fall . World No . 4 loses in straight sets to world No . 145 Aleksandra Krunic . Serena Williams through to last 16 after win over Vera Lepchenko . Andy Murray and Novak Djokovic through to last 16 in men 's draw .