Petra Kvitova became the latest top seed to fall at the U.S. Open on Saturday and Victoria Azarenka was almost the next before rallying for a third-round victory.
American Alison Riske provided the surprising upset with a 6-3, 6-0 rout of the seventh-seeded Kvitova. The 2011 Wimbledon champ became the second top seed to bow out in the first week as fourth-seeded Sara Errani lost in the second round.
The second-seeded Azarenka made sure there wasn't another stunner, coming back for a 6-7 (2-7), 6-3, 6-2 triumph over Frenchwoman Alize Cornet. The two-time Australian Open champ from Belarus was last year's U.S. Open runner-up to Serena Williams.
Azarenka will next square off against 13th-seeded Ana Ivanovic in a battle of former world No. 1s.
Ivanovic was a game away from falling to American Christina McHale, but the Serb broke serve at 4-5 in the second set to stay alive and turned the momentum into a 4-6, 7-5, 6-4 victory.
Caroline Wozniacki, the sixth-seeded Dane and 2009 U.S. Open runner-up, couldn't dodge the upset, falling 4-6, 6-4, 6-3 to Camila Giorgi during the night session at Ashe Stadium. Giorgi's next opponent will be countrywoman Roberta Vinci after the 10th seed bested fellow Italian Karin Knapp, 6-4, 6-3.
Kvitova, meanwhile, hasn't fared too well at Flushing Meadows. A pair of fourth-round results have been her best finishes in now six trips to the USTA National Tennis Center.
Riske, whose lone previous U.S. Open ended in a first-round loss in 2011, gained a measure of revenge after losing to Kvitova in the second round of last week's tune-up event in New Haven. Kvitova needed three sets to dispose of the 23-year-old Pittsburgh native last week and never found a rhythm on Saturday.
Kvitova committed seven double-faults in just eight service games and had 27 unforced errors overall, compared to just seven from Riske.
"It was really emotional for me," said Riske, who won the final eight games. "It's cool because I feel like I'm playing within myself and not doing anything too crazy. It's just really comfortable for me out there."
Up next for Riske will be Slovakian Daniela Hantuchova, who rallied to beat Israel's Julia Glushko by a 3-6, 7-5, 7-6 (7-4) score.
Another winner Saturday was 21st-seeded Simona Halep. The Romanian blasted 14th-seeded Russian Maria Kirilenko, 6-1, 6-0, to keep her brilliant summer going.
Halep has captured four titles since a first-round loss at the French Open, winning in Nuremburg, Den Bosch, Budapest and last week at New Haven. She beat Kvitova in last weekend's final and her lone three losses since Roland Garros have come against Li Na in the second round at Wimbledon, Flavia Pennetta in the second round at Bastad and Williams in the Cincinnati quarterfinals.
She'll get another crack at Pennetta in the fourth round, as the unseeded Italian followed her second-round upset of Errani with a 7-5, 6-1 win over 2004 U.S. Open champ Svetlana Kuznetsova on Saturday.
Pennetta, a three-time U.S. Open quarterfinalist, hasn't dropped a set in her three matches this week.