The 7-6 (7-2) 6-1 triumph was Osaka's first victory over a top-50 player on clay since defeating Victoria Azarenka at the 2019 French Open.
Osaka was proud of the way she maintained her mental focus after dropping her serve while serving for the first set at 5-3.
"Just being able to hang in there and eventually close it on my terms is something that I'm very proud of myself for," she said.
It's also the first time that Osaka is playing in Rome since 2019, when she reached the quarter-finals.
Osaka, who was formerly ranked No.1 but is now No.173 after a maternity break, served eight aces and produced 27 winners to her opponent's 10.
She will next face 19th-seeded Marta Kostyuk, who recently reached the final of a clay event in Stuttgart, Germany.
In other matches, qualifier Bernarda Pera beat fellow American Caroline Dolehide 7-6 (8-6) 6-3 and will next face top-ranked Iga Swiatek.
Magda Linette beat Zhu Lin 6-3 6-2, Brenda Fruhvirtova rallied past Taylor Townsend 3-6 6-2 6-0, and Lesia Tsurenko eliminated Donna Vekic 6-2 3-6 7-6 (7-5).
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