Venus Williams kept the defence of her Abierto Mexicano Telcel tournament title on track with a straight-sets win over Kaia Kanepi.
The former world number one was made to sweat by her Estonian rival who twice broke her in the opening set, but struggled on her own serve and was broken to concede the opener.
Kanepi then immediately broke Williams, but the American overpowered her to reach the last eight.
It was a good day for the top seeds with Agnes Szavay (second), Gisela Dulko (third) and Carla Suarez Navarro (fifth) all progressing to the quarter-finals.
Hungarian Szavay was too strong for Renata Voracova 7-5 6-1 while Dulko won by the same scoreline against Barbora Zahlavova Strycov.
Spain's Suarez Navarro was taken the distance by Catalina Castano 6-1 4-6 6-2 in a match that lasted two hours and 14 minutes.
Eighth seed Polona Hercog reached the last eight with a 6-2 6-4 victory over Alize Cornet.
Spanish qualifier Laura Pous Tio caused the biggest upset of the day when she beat seventh seed Roberta Vinci 6-3 6-2.
Sixth seed Sara Errani followed her fellow Italian out of the tournament at the hands of Edina Gallouitis 6-4 5-7 6-4.
Canada's Sharon Finschman, who beat fourth seed Sorana Cirstea in the opening round, continued her impressive run with a 6-2 6-3 win over Mariya Kuryttseva.