The Spanish fourth seed, however, barely broke sweat as she swept aside French wildcard Myrtille Georges 6-2 6-0 on a sun-bathed Court Philippe Chatrier.
On Monday, third seed Angelique Kerber, fifth seed Victoria Azarenka and seventh seed Roberta Vinci were knocked out in the first round."When you look at the recent tournaments, things have been very erratic and unpredictable. The players have had some tough matches," Muguruza told reporters.
"I don't think it's like the men where when you play (Novak) Djokovic you know you're going to lose. It's not the same thing in women's singles at all."
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Muguruza, who reached the quarter-finals at Roland Garros in the past two years, will next face either Russian 27th seed Ekaterina Makarova or Belgium's unseeded Yanina Wickmayer.
"I think the first rounds are the toughest ones. You have to be very motivated. You really have to play at your very best and give what you have, the best in store," she said.
And she gave some of her best tennis against Georges.
The Venezuela-born player, looking to become the first Spanish woman to lift the Suzanne Lenglen Cup since Arantxa Sanchez-Vicario in 1998, broke in the sixth game and wrapped up the opening set by winning the last 12 points.
She showed no sign of weakness in the second set, allowing her opponent just five points.
(Reporting by Julien Pretot; Editing by Larry King and Ken Ferris)
