French prospects Caroline Garcia and Kristina Mladenovic both scalped seeds at the Connecticut Open on Monday, while Eugenie Bouchard's struggles continued.
Garcia coasted past sixth seed Timea Bacsinszky 6-3 6-1 in New Haven, while Mladenovic accounted for eighth seed Sara Errani 6-2 4-6 6-4 at the WTA Premier hard-court event.
Mladenovic's win left just two seeds in the top half of the draw in the second round - Lucie Safarova (four) and Karolina Pliskova (five).
Pliskova escaped with a narrow 2-6 7-6 (7-5) 6-2 victory over qualifier Polona Hercog, while Safarova dismissed Daria Gavrilova 6-3 6-4.
Seventh seed Agnieszka Radwanska thrashed Coco Vandeweghe 6-0 6-2, but the American was not the heaviest loser on Monday - that dishonour belonged to Bouchard.
The Canadian's struggles have been well-publicised in 2015, and she enters the US Open void of confidence after slumping to a 6-0 6-1 loss to qualifier Roberta Vinci.
Bouchard has won just five matches on the WTA Tour since making the Australian Open quarter-finals.
Qualifier Yulia Putintseva dropped just four games in six sets to make the main draw, and started well against Alize Cornet - before the latter swung the match in her favour to win 2-6 6-0 6-2.
Other qualifiers Magdalena Rybarikova and Olga Savchuk were first-round winners.
Rybarikova fought back to beat Flavia Pennetta 6-7 (2-7) 6-3 6-1, while Savchuk advanced past fellow qualifier Olga Govortsova when the Belarusian retired with the score 4-6 6-3 4-0 in the Ukrainian's favour.
Savchuk's countrywoman Elina Svitolina was another to succumb to injury, with Madison Keys the beneficiary - the American progressing when leading 7-6 (7-4) 4-2.