Angelique Kerber and Petra Kvitova were both knocked out of the US Open on Saturday, leaving defending champion Sloane Stephens as the sole remaining top-six seed in the women's draw.
Wimbledon champion Kerber was beaten 3-6 6-3 6-3 by Slovak 29th seed Dominika Cibulkova in the third round.
Two-time quarter-finalist Kvitova lost 7-5 6-1 to in-form Belarusian Aryna Sabalenka.
None of this year's Grand Slam winners are now left in the women's singles.
French Open champion Simona Halep was knocked out by Kaia Kanepi in the first round, while Australian Open champion Caroline Wozniacki experienced a shock second-round loss to Ukraine's Lesia Tsurenko.
Kerber broke three times in the first set but Cibulkova repeated the feat in the second to take it to a decider, says a BBC report.
The German fourth seed, 30, was then broken again twice in the final set and misses out on a last-16 tie with last year's finalist Madison Keys.
American 14th seed Keys, runner-up to Stephens last year, recovered from a set down to reach the last 16, beating Serbia's Aleksandra Krunic 4-6 6-1 6-2.
Keys, 23, was broken in her first service game of the match en route to losing the first set but broke twice in the second set to take it to a decider.
The first five games of the third set went against serve before the American held for 4-2 and closed out the match.
"I was definitely a little bit nervous and she came out playing really well and put pressure on me from the start. I am just happy to be in the next round," Keys, 23, said.
"There were too many unforced errors in the first set but I'm glad I settled down a bit and played much better. I wasn't aggressive enough in the first set and was too passive. The biggest thing was trusting myself."
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