Top seed Victoria Azarenka was forced to dig deep as she rallied to beat Polona Hercog in three sets at the WTA Abierto Mexicano Telcel.
Azarenka would have been hoping for an easier opening-round match after overcoming Slovenian Hercog 4-6 6-3 6-1 in a two-hour battle in Acapulco on Tuesday.
Playing for the first time since losing to eventual champion Angelique Kerber in the quarter-finals of the Australian open last month, Azarenka was slow out of the blocks as Hercog earned control of proceedings.
Azarenka double-faulted on four occasions and won just 52 per cent of her first service points in a disappointing opening set.
But the two-time major winner lifted her game in the second set by winning 92 per cent of points on her first serve as she levelled the match.
Azarenka raced away with the contest in the third, firing down two aces as Hercog tallied five double faults to win through to the second round.
Awaiting Azarenka in the second round is 2014 champion Dominika Cibulkova, who posted a double bagel.
Cibulkova - the 2014 Australian Open runner-up - barely raised a sweat as she blitzed Lourdes Dominguez Lino 6-0 6-0 in less than an hour.
Christina McHale, who lost out to Cibulkova in the final two years ago, also advanced to the second round after fighting back to defeat Heather Watson in three sets.
McHale dropped the opening set but rallied to eliminate the Brit 4-6 6-0 7-6 (7-1).
Danka Kovinic is next up for McHale after the seventh seed accounted for Tatjana Maria 7-6 (8-6) 6-2.
In other results, second seed Sloane Stephens defeated fellow American Louisa Chirico 1-6 6-3 6-3, Shelby Rogers trumped Anna Tatishvili 7-6 (8-6) 6-0, Julia Glushko accounted for Ana Sofia Sanchez 6-4 6-1 and Olga Govortsova ousted Lara Arruabarrena 6-1 6-2.
Mirjana Lucic-Baroni downed Irina Falconi 6-2 6-3 and Samantha Crawford lost out to Monica Puig 6-4 6-4.