It was also the first matchup since the Open Final in 2018.
Williams fired 12 aces and didn t face a break point as she beat Osaka for the first time in three meetings - avenging her shock loss in past year s controversial US Open final in which the American star was docked a point and a game after losing her temper when warned about coaching.
There was no repeat of the scenes that marred last year's Grand Slam final in NY when Williams was docked a game after clashing with umpire Carlos Ramos, calling him a "thief" and accusing him of double standards that favored male competitors.
Up next for Williams will be the victor of the late match between holder Simona Halep and Czech qualifier Marie Bouzkova.
The 23-time grand slam champion has since apologised and, in their first meeting since, Williams delivered an impeccable performance at the WTA Premier tournament to score her first career win over Osaka.
The Czech qualifier continued her run, winning the first set 6-4 when Halep retired injured.
"I knew her game a little bit more, so that's a little easier, and, you know, I'm just overall a little bit better".
Osaka said she played too defensively - something she said was hard to avoid when Williams was hitting 31 winners.More news: Bayern Munich sign Ivan Perisic on loan from Inter Milan
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"We haven't played since NY, which was a really good match for her", Williams said of Osaka on Tennis.com.
"I m getting there", said Williams, who hasn t dropped a set this week as she pursues a fourth Toronto title to go with those she won in 2001, 2011 and 2013.
"In the third set I told myself, Go big or go home, " the home hope said. "It's definitely not where I want to be, but I'm getting there".
Osaka will go from No. 2 to No. 1 on Monday when the new ranking is announced.
"I think in the first set we were kind of just serving it out, and then I was the one that - I don't even think I dipped".
Meanwhile, Osaka, 21, explained that Williams "is someone that I looked up to forever".
The Romanian, who roared back from 0-4 down to level the set at 4-4 only to lose the last two games, said she had been bothered by her Achilles tendon from the start of the tournament.
The other semifinal will feature American Sofia Kenin (29th) against Canadian tennis star Bianca Andreescu (27th).