But there were no such problems for men’s top seed Novak Djokovic and eighth-seeded Andy Murray, both former champions in New York, who remained on course for a mouth-watering quarter-final showdown.
Five-time women’s champion Serena Williams was also a comfortable winner, racking up her 75th victory at the US Open.
Third seeded Kvitova lost her third round tie 6-4, 6-4 to pint-sized 21-year-old Krunic, who came through qualifying and had never won a match at a major before this year’s event.
Kvitova joins second seed Simona Halep, fourth-seeded Agnieszka Radwanska, Angelique Kerber, the sixth seed, and eighth-seeded Ana Ivanovic in failing to make the fourth round.
The 24-year-old Kvitova dropped serve five times and committed 34 unforced errors against the 5ft 4in (1.63m) Krunic who divides her time between the pro tour and studying for a university economics degree.
“It was an honour to play on the same court as Petra, she’s a great champion. I watched both her wins at Wimbledon,” said Krunic, who next plays former world number one Victoria Azarenka, the runner-up to Williams in the last two years.
Kvitova said she was taken by surprise by her opponent’s poise and power.
“I didn’t really expect how she played so well. Suddenly she just pushed the ball very hard,” said the Czech.
Djokovic, the 2011 champion, brushed aside America’s Sam Querrey for the eighth time in nine meetings, with a 6-3, 6-2, 6-2 win taking the world number one into the fourth round at a major for the 22nd consecutive time.
“Sam is big server, very powerful. But he doesn’t move that well so I wanted to keep him moving around the court, mix up the pace and get as many returns as possible back,” said Djokovic after an 85-minute win which was never in doubt from the time he raced into a 5-0 lead in the first set.
- Last American man -
Djokovic will next face John Isner, the last American left in the draw, if the 13th seed can beat Germany’s Philipp Kohlschreiber.
Women’s top seed and double defending champion Serena Williams eased past her third successive American compatriot, Uzbekistan-born left-hander Varvara Lepchenko, 6-3, 6-3 and will tackle tough Estonian Kaia Kanepi for a quarter-final berth.
“I had to make some adjustments because of the wind but I got some excellent advice from my coach,” said 32-year-old Williams, playing in her 15th US Open and aiming to become just the second woman after Chris Evert in the 1970s to win three New York titles in succession.
“My opponent played at an unbelievable level but I tried to hang in there and do what I needed to do.”
Murray, the 2012 champion, hit 47 winners in a 6-1, 7-5, 4-6, 6-2 victory over Russia’s Andrey Kuznetsov, the world 96, to book a seventh appearance in the fourth round.
“I got off to a good start and then had a cushion when he got back into the match in the third set,” said 27-year-old Murray.
Also going through to the last-16 for a third successive year was Canadian fifth seed Milos Raonic who beat Victor Estrella Burgos, the 34-year-old tournament debutant from the Dominican Republic, 7-6 (7/5), 7-6 (7/5), 7-6 (7/3).
He will face Japanese 10th seed Kei Nishikori who reached the fourth round for the first time since his 2008 debut with a 6-4, 6-2, 6-3 victory over Leonardo Mayer of Argentina.
Later Saturday, Australian Open champion Stan Wawrinka, the third seed, faces Slovenia’s Blaz Kavcic, the world number 92, who has never defeated a top-10 opponent in 10 attempts.
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