Latvian Anastasija Sevastova followed up her shock win over top seed Jelena Jankovic by knocking out Czech player Iveta Benesova 6-1 6-2.
Second seed Daniela Hantuchova stepped up her bid for a first title since 2007 by racing through to the Monterrey Open quarter-finals.
Slovakian Hantuchova, 26, was far too strong for Estonia’s Kaia Kanepi in their second-round match and won 6-1 6-3 to set up a clash with American Vania King in the last eight.
Hantuchova’s compatriot Dominika Cibulkova also advanced to the quarter-finals, albeit by a more difficult route, with the fourth seed coming from behind to beat Italian Sara Errani 3-6 6-4 6-1.
Hungarian Agnes Szavay, seeded fifth, also needed three sets to progress as she edged past Germany’s Anna-Lena Groenefeld 6-1 6-7 (7/4) 6-1.