World number two and US Open second seed Caroline Wozniacki admitted her shock at losing to Lesia Tsurenko in the second round.
Following world number one Simona Halep’s first-round exit, the US Open has now lost its top two women’s seeds after Wozniacki went out to the Ukrainian, the world number 36, 6-4 6-2.
Danish star Wozniacki, who has reached the US Open final twice, said it was an unusual feeling to be knocked out at Flushing Meadows so early.
“She played the game I was supposed to play,” she said.
“This part of the season is usually a part of the season I really look forward to, one where I really play well.”
Meanwhile, fourth seed Angelique Kerber eventually saw off Sweden’s Johanna Larsson in three sets, winning 6-2 5-7 6-4, and she will face Dominika Cibulkova in the last 32, who beat Taiwan’s Hsieh Su-wei, also in three sets.
Elsewhere, Madison Keys, Petra Kvitova, Aliaksandra Sasnovich and Naomi Osaka all won to seal their passage into the next round.