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The Czech would likely need to win the year-ending championships in Shenzen combined with a poor run from Barty for this to happen.
Third-ranked Osaka and No.4 Elina Svitolina nominally remain in contention to top the rankings but would need a lot to go their way.
Things looked on track for Barty who started the final in composed fashion as her rival mixed sublime shot-making with unforced errors.
Silver service: Ash Barty poses with her runners-up trophy.Credit:AP
After five deuces on Osaka's serve in the sixth game, Barty made the first breakthrough to take a 4-2 lead.
She eventually served out the set to love and looked to be in good shape to take her career record to 3-1 against Osaka before the 2018 US Open mounted a comeback.
Osaka's returns began to find their range more and she broke for a 4-2 lead in the second set before serving it out to take the clash into a decider.
Naomi Osaka fought back after dropping the opening set of the decider in Beijing.Credit:AP
With three consecutive three-setters behind her in Beijing, including the semi- final against Kiki Bertens where she saved a match point, Barty appeared to tire in the third set.
Nineteen unforced errors in the final set took their toll on this year's French Open winner, who was broken twice in the final set for Osaka to close out the match for her third title of 2019.