The 21-year-old, who will play Karolina Pliskova for a place in the final, committed a string of unforced errors but still proved too strong for the sixth-seeded Ukrainian.
Osaka wrapped up the win in an hour and 12 minutes to become the first Japanese woman to reach the last four at Melbourne Park since Kimiko Date in 1994.
Osaka's first Grand Slam triumph at the U.S. Open last year was overshadowed by a row between Serena Williams and umpire Carlos Ramos but there will be no rematch of that match at Melbourne Park.
Williams, a 23-times major winner, squandered four match points and blew a 5-1 lead in the decider to fall to seventh seed Pliskova 6-4 4-6 7-5 in the last quarter-final of the women's draw. [L8N1ZN0BY]
"I have played her (Pliskova) a couple times already. She's really tough to play," said Osaka. "I can barely read her serve, so it's very difficult for me.
"I see now that she doesn't hit too many unforced errors, which is a little bit dangerous."
Osaka said Pliskova always seemed to stay calm on court, something she struggles with.
"For me, today, I had one goal - it was to try as hard as I can and not get angry. I didn't do that well in the past two rounds, but I played well today," she added.
INCONSISTENT FIRST SET
Osaka's power was on full display against WTA Finals champion Svitolina, with the Japanese thumping 31 winners to her opponent's 11. But she was also sloppy at times, committing 25 unforced errors, nine more than Svitolina.
"Everyone knows my backhand was a little bit inconsistent in the first set," Osaka said. "I think I was making too many unforced errors on the serve.
"So in the second set I just tried to play more and see if she's gonna hit winners or not."
Ukrainian Svitolina, 24, kept her errors in check to get two service breaks back and stay in the opening set at 4-5.
Osaka blew three set point opportunities on Svitolina's next serve with three embarrassing errors, berating herself with angry slaps on the thigh.
However, the Japanese gathered herself and hit a crisp backhand to set up a fourth chance, and converted it when Svitolina thumped a return into the net.
Svitolina took a medical timeout in the second set for neck and shoulder problems with Osaka up 3-0 but she was broken again after returning to the court.
"I felt pain from the beginning of the tournament here and there. It was going and coming back," Svitolina said.
"Unfortunately today was a little bit worse than I expected.
"It's normal to have some tension during the Grand Slam and it happens, but unfortunately I couldn't handle it today."
Fourth seed Osaka served her eighth ace then converted her first match point with an overhead winner to seal the win.
(Editing by Peter Rutherford)
