Barty joined the Japanese sensation at the top of Red Group with a 5-7, 6-1, 6-2 victory over Belinda Bencic in one hour and 56 minutes. The result secured the year-end No.1 ranking for the Australian.
"I gave Belinda too many looks at second serves in the first set," Barty told reporters. "That was the key for the second and third sets ... try to dominate a little bit more with the first serve."
In-form Osaka outlasted Petra Kvitova 7-6 (7/1), 4-6, 6-4 in two hours and 39 minutes to notch her first victory at the WTA Finals after a winless campaign in her debut appearance last year. She stretched her current winning streak to 11 matches having claimed titles in Osaka and Beijing.
In a rematch of this year's tense Australian Open final, world No.6 Kvitova came out with intent until she stuttered in the eighth game of the opening set with three double faults to squander the game.
Osaka held off two break points in the ninth game before dominating the tiebreak to draw first blood.
It seemed to shake the Czech's confidence but Kvitova, the oldest player in the draw, impressively clawed back in the second set and took advantage of an error-strewn Osaka who several times slammed her racquet in frustration.
A refocused Osaka put the foot down in the deciding set but could not convert two match points in the eighth game before finally closing it out in the 10th.
"It definitely feels good to win," Osaka told reporters.
"I didn't play well in the (WTA) Finals last year, so I'm kind of just trying to redeem myself a little bit in that sense."
Kvitova rued her wayward serving.
"I just wanted to have a good second serve. Sometimes it didn't really pay off as I wished," she said.
In the later match, Barty came back from a torrid first set marred by 20 unforced errors and being unable to crack Bencic's confident serve.
The French Open champion rediscovered her consistency with a blistering second set, breaking the world No.7 twice and levelling the match with an ace.
Bencic received medical treatment on her left heel after the second set and struggled for the remainder of the one-sided match.
"It was bothering me a little bit. Hopefully I can take a day off tomorrow," the 22-year-old said.
Bencic had leapfrogged Serena Williams and secured the last ticket to Shenzhen after claiming the title in Moscow last week.
"To play against the best of the best, every match is high quality and it's about coming out here and bringing the best each time," Barty said after the match.
The Purple Group start on Monday with world No. 2 Karolina Pliskova playing defending champion Elina Svitolina, while US Open champion Bianca Andreescu takes on Wimbledon winner Simona Halep.
The top two players in each group advance to the semi-finals next Saturday.
The event's move to the southern Chinese city of Shenzhen, after a five-year run in Singapore, is highlighted by a record prize pool in tennis with the winner receiving up to $4.725 million.
The agreement is the first bilateral trade deal between the Gulf and South America