Ekaterina Makarova closed out a marathon day at the US Open with a surprise triumph over fifth seed Caroline Wozniacki as the Russian registered a first win in eight meetings against the former world number one and a place in the third round.
After rain washed out most of Tuesday's play, there was a bumper schedule at the Billie Jean King National Tennis Center as officials raced to get back on track and the 40th-ranked Makarova brought the curtain down with 6-2 6-7(5) 6-1 win.
Twice a runner-up at Flushing Meadows, Wozniacki arrived in New York ready to claim an elusive first grand slam crown and add some sheen to a frustrating season that has seen the 27-year-old reach six finals but fail to win a single one of them.
Garbine Muguruza is through to the third round for the first time in her career after beating Duan Ying Ying 6-4 6-0.
The Spaniard is in an incredible six-way battle for the women's number one ranking - down from eight when the tournament got underway after Johanna Konta and Caroline Wozniacki fell.
Highlights: Garbine Muguruza powers past Duan Ying Ying
Canada's Eugenie Bouchard suffered a 7-6 6-1 defeat to Russia's Evgeniya Rodina in the US Open first round at Flushing Meadows.
Rodina beat 2014 Wimbledon runner-up Bouchard at Arthur Ashe Stadium and will face fourth seed Elina Svitolina in round two.
Bouchard was back on centre court on Wednesday, the United States Tennis Association saying it held no grudges against the Canadian who is suing the organisation for negligence over injuries suffered in a locker-room fall.
Usually the marquee matchups of the day are reserved for Arthur Ashe Stadium and a contest between two players barely ranked inside the top 100 seemed an unconventional choice - even more so given the lawsuit now before the courts.
"I was surprised," admitted the 76th-ranked Bouchard following a 7-6(2) 6-1 thrashing by 89th-ranked Rodina. "But it's always an amazing opportunity to play on the biggest tennis court in the world."
Bouchard crashes out to Rodina as disappointing season continues
Local hope Sloane Stephens outlasted Slovakia's 11th seed Dominika Cibulkova 6-2 5-7 6-3 to reach the third round, while American CoCo Vandeweghe came back from a set down to claim a 2-6 6-3 6-4 first-round victory over compatriot Alison Riske.
Vandeweghe, coached by 1987 Wimbledon champion Pat Cash, fired 31 winners to book a second-round meeting with Tunisia's Ons Jabeur.
Eighth-seeded Russian Svetlana Kuznetsova saved three match points before securing a 4-6 6-4 7-6(2) first-round victory over Czech Republic's Marketa Vondrousova.
Elsewhere, 13th seed Petra Kvitova advanced to the third round after beating Alize Cornet 6-1 6-2.
And Ekaterina Makarova closed out a marathon day with a surprise triumph over fifth seed Caroline Wozniacki as the Russian registered a first win in eight meetings against the former world number one and a place in the third round.
After rain washed out most of Tuesday's play, there was a bumper schedule at the Billie Jean King National Tennis Center as officials raced to get back on track.
Results from the US Open Women's Singles matches on Wednesday