Second seed Dominika Cibulkova was ousted in the Pattaya Open first round in Thailand on Tuesday, while top seed Vera Zvonareva progressed.
The Slovakian world number 16 was thrashed 6-4 6-1 by Briton Anne Keothavong in one hour and 22 minutes.
The result marked Keothavong's first-ever win over a top-20 player on the WTA Tour, and she will be rewarded with a second-round date against India's Sania Mirza, who beat Ayumi Morita 3-6 6-1 6-2.
Zvonareva - a two-time champion in Pattaya City - accounted for Thai qualifier Varatchaya Wongteanchai 6-2 7-5, and the Russian will take on Akgul Amanmuradova in the second round.
Amanmuradova, of Uzbekistan, was handed a comfortable passage through to the final-16, when her opponent Laura Robson, of Great Britain, retired due to a wrist injury when trailing 5-2.
Russian fourth seed Maria Kirilenko saved a match point before fighting back to beat Australia's Casey Dellacqua 4-6 7-6 (8-6) 6-3 in a battle lasting two-and-a-half hours.
Kirilenko will meet Dellacqua's compatriot - Anastasia Rodionova - in the next round.
Third-seeded Slovakian Daniela Hantuchova thrashed Thai wildcard Nungnadda Wannasuk 6-2 6-0 to earn a date with Poland's Urszula Radwanska, who accounted for Russian Anastasia Pivovarova 6-4 6-1.
Fifth seed Zheng Jie fell to Taipei's Su-Wei Hsieh 6-3 6-2, with the qualifier to meet Japanese veteran Kimiko Date-Krumm next, after she beat Thai wildcard Nicha Lertpitaksinchai 6-1 6-1.
Tamarine Tanasugarn was the only local to progress to the second round on Tuesday, moving through when sixth seed Galina Voskoboeva retired injured with the Thai leading 6-3 4-2.
American eighth seed Vania King beat Great Britain's Heather Watson 6-4 6-2 to set-up a clash with Russian Alla Kudryavtseva, who defeated qualifier Zhou Yi-Miao 6-1 7-5.
Japan's Misaki Doi eased past Thai wildcard Noppawan Lertcheewakarn 6-1 6-3, to book a meeting with seventh seed Sorana Cirstea.
Taipei's Kai-Chen Chang passed Eleni Daniilidou 7-5 6-1, to advance to a second-round match with Tanasugarn.