Jabeur led Elena Rybakina 6-4, 3-2 when Rybakina retired due to illness.
Jabeur, the No. 16 player in the world, converted all three break-point chances to take the first set. From Tunisia, Jabeur became the first Arab tournament winner in WTA history in June at Birmingham, England. She has a WTA Tour-high 44 match victories this season.
The ninth-ranked Muguruza advanced without playing a point after Marketa Vondrousova withdrew in the morning because of a gastrointestinal illness.