The 28-year-old won the final of the Korea Open 6-4 6-2 on Sunday, beating Joint in their first meeting on the WTA tour and the first all-Australian singles final on the tour since 2017.
Joint started brightly and secured an early break to take a 2-0 lead, but Birrell quickly turned the match and broke the 20-year-old twice on her way to claiming the opening set 6-4.
The world No.60 Birrell capitalised on Joint's 41 total unforced errors in the final and converted six of her 10 break point opportunities as she wrapped the match up in straight sets in 83 minutes.
Birrell is expected to now enter the top-50 in the world rankings for the first time.
In Singapore on Sunday, Canada's Leylah Fernandez beat Australia's Talia Gibson 7-5 6-0 to clinch the Singapore Open title and claim the sixth WTA title of her career.
The world No.31 Fernandez pulled away after breaking early in the opening set, but Gibson refused to fade, twice clawing back from a break down to stay level. Fernandez, 24, steadied herself in the closing stages and converted her first set point to take the opener 7-5.
Fernandez then dominated the second set, racing into a double-break lead as the 22-year-old Gibson's challenge faded. The Canadian eventually wrapped up a commanding 6-0 set to seal the title.
Gibson, who advanced to her first WTA final after her semi-final opponent Tatiana Prozorova was forced to withdraw, missed out on claiming her maiden title.
With Reuters
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