The Romanian was forced to save four break points in her opening service game but went on to claim 12 consecutive games to avenge her Indian Wells loss to eventual champion Osaka in March.
The 20-year-old Osaka failed to convert any of her six break points before conceding the match in just under an hour. “I just tried to build my points, to build a little bit of space and open the court, and tried to make winners,” Halep said.
“It’s tough to make winners on this court, for me, but it’s good when I can build the point. I just want to play like today.” Halep will next meet American Madison Keys, who clinched a 7-6(2) 7-6(0) win over Croatian qualifier Donna Vekic.
World number two Caroline Wozniacki began her campaign with a comfortable 6-1 6-4 victory over Belgian Alison van Uytvanck.
The 27-year-old Dane will face 15th seed Anastasija Sevastova for a place in the quarter-finals. A decade after they last met in Rome, Maria Sharapova once again did enough to secure a victory over Dominika Cibulkova in three sets. Sharapova struck 41 winners as she rallied back from a set down to edge her opponent 3-6 6-4 6-2 in a two and a half hour contest.