After only one seed fell on day one of the US Open, No.31 seed Yulia Putintseva losing to Kaia Kanep, day two saw another upset as Spaniard Sorribes Tormo beat No.22 seed and Australian Open semi-finalist Karolina Muchova, 6-2, 7-6(3).
Tormo dominated the first set but was made to fight for the second, coming back from 3-5 down to seal a straight-sets win.
She will face Hsieh Su-Wei in the second round.
Anastasia Pavlyuchenkova shruged off a late arrival in New York to Alison Riske, 6-4, 6-2.
The French Open finalist was stuck in Canada due to visa issues and arrived in New York on Saturday.
She will face Anna Karolina Schmiedlova in the second round.
Romanian Sorana Cirstea confirmed she is a form player with a 7-6(5), 3-6, 6-0 defeat of No.29 seed Veronika Kudermetova.
Cirstea will face Shelby Rogers for a spot in the third round.