Two-time grand slam champion Osaka was coming off her victory on home soil at the Pan Pacific Open and beat US Open champion Bianca Andreescu
and title holder Caroline Wozniacki on the way to the final.
The early omens were not promising for the 21-year-old as she served up two double faults on the way to being broken in the sixth game and losing the opening set.
The fightback for Osaka started in the sixth game of the second set as Barty hit a forehand long to trail 2-4.
Osaka held serve to level at one set apiece and an immediate break at the start of the second left her in control.
It was the third title of 2019 for Osaka and fifth overall, but she revealed that she needed to give herself a pep talk after her early struggles in the final.
“Basically I was telling myself to keep fighting even though my attitude was really trash. This is a final, I’m really privileged to be here in the first place, so keep fighting,” she said.
The pair will resume their budding rivalry at the WTA Finals in Shenzhen later this month, Osaka clinching her place in the $14 million showpiece earlier this week.
In the men’s ATP tournament, top seed Dominic Thiem also had to come from a set down to beat Stefanos Tsitsipas of Greece, 3-6 6-4 6-1 in the final.
It was the 15th ATP career title for Austria’s Thiem, who beat former world number one Andy Murray on the way to the title.
Djokovic eyes Olympic gold
There was better luck for men’s world number one Novak Djokovic in the final of the Japan Open, the Serbian ace dispatching Australian qualifier John Millman 6-3 6-2 in the final.
It was his first tournament since pulling out of the US Open with a shoulder injury and Djokovic signaled he is set to finish the season in style, with the ATP Finals in London next month.
But Djokovic has another longer term goal, deliberately playing in Japan for the first time so he could experience conditions at the Ariake Colosseum, a venue for the Tokyo 2020 Olympics.
Djokovic wants to add Olympic singles gold to add to his 16 grand slam titles and this was an important first step for him.
“I am very glad now, seeing and experiencing how I play (in Ariake) because I feel comfortable. It feels great playing on this court… I am hoping that can also be the case in the Olympics,” he said.
Next up for Djokovic will be his defense of the Shanghai Masters title, where he is top seed.