The No.3-seeded Svitolina looked like a woman on a mission early on in the match, building leads of 4-1 and 5-3 and, after letting those slip, even getting to double set point at 6-4 in the tie-break.
But the No.1-seeded Stosur saved both set points and ended up running off four points in a row for the set, breaking in the opening game of the second set and never looking back to win, 7-6(6), 6-2.
"I have to use my experience, and maybe everything I've learnt over the years, to beat these younger players," Stosur said. "When I was in that position playing the older players, I'd feel I had nothing to lose - your ranking's not as good, you're younger, you're not expected to win, so you go out there and really go for it. If you win that's fantastic, and if you lose you weren't expected to win anyway.
"Now I have to make sure I'm playing my best tennis against anyone. Everyone's got the ability to beat anyone. You can't take anything for granted, even when your opponent is 20 or 21 years old."
Stosur was asked whether she thinks there's a changing of the guard on the tour right now.
"Every few years you're going to have a group of players who are going to break through and move higher and higher up the rankings, and every year or so some of the older players will retire, and all of a sudden there's a new-looking Top 10 or Top 20. That's just the evolution of the sport, really.
"I think it's exciting for tennis at the moment that there are so many younger players doing well, so it's kind of like a battle between the younger ones and older ones like me on the tour right now."
The Australian touted one of her own generation as the best of the best, though.
"She didn't dominate as much this year, but Serena's still clearly No.1 at the moment," she added.
The Japan Women's Open Tennis is the only tournament Stosur has won more than once in her career - she's won it twice, in 2009 and 2013 - and now she's a win away from upping that to three times.
Standing between Stosur and the title will be the No.5 seed, Zarina Diyas, who hung on to win an all-WTA Rising Star semi against Luksika Kumkhum, 6-2, 7-5. Diyas originally led 6-2, 4-1 before Kumkhum started hitting the targets on her two-fisted groundies - but Diyas managed to close it out.
Diyas was playing her first WTA semifinal - now she's through to her first WTA final.
Stosur and Diyas will be playing each other for the first time.