The four-time Grand Slam champion beat unseeded American Hailey Baptiste 6-7 (2/7), 6-1, 6-2 in another confidence boost ahead of the Australian Open in Melbourne.
Osaka said her hard work was starting to pay off after booking a place in the final four of a WTA event for the first time since the Miami Open in April 2022.
The Japan star gave birth to daughter Shai in July the following year, midway through a 15-month break, before returning to the tour 12 months ago.
"I put a lot of work in throughout last year and even though the results didn't show it, I'm just continuing to try as hard as I can to see where it gets me," the 57th-ranked Osaka said.
"The key focus again today was just to have a lot of belief and confidence in myself."
As with her first two matches in Auckland, the 27-year-old lacked rhythm in the opening set.
She unearthed a powerful serving game over the next two sets, ending with nine aces to give her 21 for the tournament.
World number 92 Baptiste sought treatment for an ankle injury during the second set.
Osaka, a two-time Australian Open champion, admitted that helped her cause.
"She's an amazing player, it sucks that she was injured but I think we played a really high-quality match," said Osaka, who is seeded seventh in Auckland.
"In a weird way I'm glad to play a three-set match because I think that's the kind of experience that I need."
Osaka's semi-final opponent will come from the winner of a quarter-final later on Friday between eighth seed Katie Volynets and unseeded American compatriot Alycia Parks.
