Seventh-seeded Muguruza saved a set point in the first set and needed a medical timeout before advancing with a 7-5, 6-4 win over Marina Erakovic.
Olympic gold medalist Monica Puig raced through her opening match, beating Patricia Tig 6-0, 6-1. Duan Yingying was among the early winners with a 6-3, 6-4 win over Rebecca Sramkova.
Australian teenager Destanee Aiava’s milestone match ended in a 6-3, 7-6 (4) loss to German qualifier Mona Barthel. The 16-year-old Melbourne high school student became the first player born in this millennium to play a main draw match at a major.
Jeremy Chardy was the first winner on the men’s side, and only played four games. He was leading 4-0 when Nicolas Almagro retired from their match with an injured right calf muscle.