Wimbledon champion Petra Kvitova became the fifth top 10 seed to exit the US Open in the first week when she was knocked out by Serbian world No.145 Aleksandra Krunic on Saturday.
Five-time women's champion Serena Williams was a comfortable winner, racking up her 75th victory at the US Open, while Wimbledon runner-up Eugenie Bouchard beat determined Czech Barbora Zahlavova Strycova.
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 No.1 Victoria Azarenka, the runner-up to Williams in the past 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.
Williams, who has so-far failed to add to her cache of 17 grand slam titles this year, is taking her last chance to do so seriously.
Although she hasn't won a major this year, Williams arrived in New York having won her fourth and fifth titles of the year in the weeks before the Open at Stanford and Cincinnati.
She dropped just five games in getting past compatriots Taylor Townsend and Vania King in the first and second rounds, but found the going tougher in a 6-3 6-3 win over Varvara Lepchenko, a player she knows well and sometimes trains with.
"My opponent today was spectacular," Williams said.
"It was a really tough match and it just shows how far Varvara has come - I'm really proud of her."
But it's Williams who is going on to face Estonian Kaia Kanepi for a quarter-final berth.
Williams is one of only three of the top eight women's seeds remaining in the tournament.
The 20-year-old seventh seed Bouchard from Canada overcame 39 unforced errors on a windy Arthur Ashe Stadium court to beat Strycova 6-2 6-7 (2-7) 6-4 and book a last-16 clash with Russian 17th seed Ekaterina Makarova of Russia.
"To just give away so many free points is really frustrating to me," Bouchard said.
"So I was definitely disappointed with that. I realised in the third I can still stay with her, I don't have to go for crazy shots all the time. I think I was a bit too impatient, basically, in the second set."
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