A wrist injury forced top seed Victoria Azarenka to pull out of her second-round match against Dominika Cibulkova at the Abierto Mexicano Telcel on Wednesday.
Azarenka was coming off a gruelling three-set win over Polona Hercog in her first match since losing to eventual champion Angelique Kerber in the quarter-finals of last month's Australian Open.
The world number 14's withdrawal due to a left wrist injury handed 2014 champion Cibulkova a walkover and spot in the quarters of the WTA tournament in Acapulco.
"I don't want to have a setback," Azarenka said. "Done it too many times before.
"It didn't get me better. Trying to listen to my body, be smarter."
Next up for Cibulkova is sixth-seeded Swede Johanna Larsson, who overcame Shelby Rogers 6-1 6-7 (2-7) 7-5.
Sloane Stephens - the second seed - continued her good form with a comfortable straight-sets win over Olga Govortsova.
Stephens accounted for Govortsova 6-4 6-1 to reach the last eight, taking her win-loss record in 2016 to 8-1, which includes the ASB Classic title.
The American's only defeat this year came at the Australian Open, where she was eliminated in the first round.
Stephens' victory earned her a showdown with Japanese wildcard Naomi Osaka, who downed Mariana Duque-Marino 6-3 7-6 (7-5).
Russian third seed Anastasia Pavlyuchenkova also booked a quarter-final berth via a 6-2 6-1 win against Julia Glushko.
Pavlyuchenkova will now meet eighth seed Yanina Wickmayer, who prevailed 3-6 6-3 6-3 against Monica Puig.
Johanna Konta was not so fortunate, the fourth seed and Australian Open semi-finalist stunned 4-6 6-2 7-5 by Mirjana Lucic-Baroni.
Awaiting Lucic-Baroni in the quarters is Christina McHale, who rallied to beat seventh seed Danka Kovinic 3-6 6-3 6-2.