Fourth seed Kirsten Flipkens suffered a shock first-round exit at the Monterrey Open in Mexico, while Caroline Wozniacki made brisk progress.
Belgian Flipkens entered the tournament as a wildcard but was unable to seize her opportunity as she was beaten 6-4 1-6 6-2 by American qualifier Julia Boserup.
There was no such trouble for third seed Wozniacki, though, as the Dane continued her good form of late to claim a 6-3 7-6 straight-sets victory over Coco Vanderweghe.
Big-serving Vanderweghe held the former world No 1 in the opening exchanges before Wozniacki won five straight games from 3-3 to take control of the match and progressed after winning a second-set tie break.
Fellow seed Monica Puig won the first 11 games on her way to a 52-minute 6-0 6-3 win over Olga Puchkova, while Magdalena Rybarikova made it through to the second round with a 6-3 3-6 6-2 victory over Olga Govortsova.
There was also success for Thai youngster Luksika Kumkhum, who claimed a 7-5 6-0 win over Slovenia’s Dalila Jakupovic.
Canadian qualifier Aleksandra Wozniak came back from a set behind to move past home Mexican wild card Marcela Zacarias 3-6 6-0 6-1.