Barty's 5-7 6-1 6-2 win over Bencic on Sunday night means the 23-year-old will become the first female Australian to finish a tennis season in top spot since rankings were introduced in 1973.
The only way she cannot is if she fails to walk on court again this week and Karolina Pliskova wins the competition, which boasts the biggest cheque ever offered in men's or women's tennis.
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The winner will receive at least $4.1m (£2.2m) - or $4.7m (£2.5m) should the champion go unbeaten en route to the title.
Bencic took the first set by breaking the Australian in the penultimate game as Barty sent her forehand long from behind the baseline.
Barty responded in style, however, reeling off five straight games to take control of the second set as Bencic struggled with a heel injury.
The world No 7 returned after a medical time-out to win the opening game of the final set but Barty was soon in control again, securing a break in the third game that allowed her to cruise to victory.
"It's been an incredibly consistent year across all surfaces, across all continents in the world," said 2019 French Open champion Barty post-match.
I feel like (I've) played an exceptional level of tennis.
OSAKA SEES OFF KVITOVA
Osaka defeated Petra Kvitova 7-6 (1), 4-6, 6-4 in the first of three round-robin matches at this year's WTA Finals, extending her winning-streak to 11 matches.
Osaka, fresh off title wins in Osaka, Japan, and Beijing, threw away two match points when serving at 5-2 in the third set, but sealed the win on her third match point.
The result means the Australian Open champion, who beat Kvitova in the final back in January, has got her red group round-robin series off to the perfect start.
Osaka's lost all three of her round-robin encounters on her debut in last year’s edition in Singapore.
29-year-old Kvitova, the oldest player in this year's eight-player competition, is competing in her seventh WTA Finals.
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