Playing her first event since losing the Wimbledon final, Williams faced Osaka for the first time since losing to her previous year in the U.S. Open final.
It was their first matchup since the 2018 US Open final, when Williams' dispute with the umpire overshadowed Osaka's upset.
But with the year's fourth grand slam just around the corner, Williams showed she still had the game to claim a 24th major title by defeating Osaka 6-3 6-4.
Unlike at Flushing Meadows past year, where Osaka won her first Grand Slam title, there were no fiery exchanges with the umpire, no smashed rackets, no point or game penalties - just a rock-solid performance from Williams.
Williams next faces either fourth-seeded Simona Halep ― who beat her in the Wimbledon final in July ― or Czech qualifier Marie Bouzkova.
Since then, the 21-year-old Osaka has only brightened her star in the tennis world, rising to No. 1 in the WTA World rankings heading into Friday.
"I knew [Osaka's] game a little bit more, so that's a little easier", Williams said.
Playing her idol once again, Osaka underwent the same sensations when she played her for the first time in Miami, Florida.
"So in a weird way, losing today I accomplished my dream", she said, according to CNN.
"I just wanted to come out and try to win a set this time, because she's beaten me twice", Williams said, via tennis.com.
"I honestly don't really think there was ups and downs", Osaka said. "So I was just was like "I gotta keep going". "It s definitely not where I want to be, but I m getting there". "I don't really like too much change".
"But I think there were chances where she hit shorter balls, but I wasn't expecting her to hit the shorter balls, so I didn't move up on time", she said.
Pliskova, number one for eight weeks in 2017, needed to win the tournament to regain top spot but lost 6-0 2-6 6-4 to Canadian teenager Bianca Andreescu.
But the 19-year-old, who shot to prominence with her Indian Wells triumph in March, went toe-to-toe with her more experienced opponent to win by serving out to love.
A win for Williams sees her bounce-back after losing in disastrous conditions at the Arthur Ashe Stadium in NY, when the 37-year-old received code violations for coaching, smashing her racket, and calling the umpire Carlos Ramos a "thief" at the 2018 US Open.
The victor will proceed to the final of the Rogers Cup - the Canadian Open, a Masters event worth 1,000 ranking points - and face the victor of the other semifinal, Sofia Kenin or Bianca Andreescu.