Bouchard will look to make it seven on Friday as she takes on another German: Top seed Julia Goerges for a spot in the final.
The world number 9 bested Swiss qualifier Belinda Bencic in a tightly contested two-setter, taking out the final 6-4, 7-5.
The Swiss qualifier claimed a 6-2, 3-6,7-6 (5) win to end Yastremska's wonderful run which saw her burst onto the scene by winning the Hong Kong Open last week.
The German won her first tournament of the year in Auckland, reached another final at Charleston in April, and the Wimbledon semifinals.
Thousands rally for Taiwan independence vote
China has tolerated Taiwan's autonomy - and US arms sales to the island - so long as both sides respect there is only "one China". While the organisers said on Saturday that 120,000 people took part in the rally, police said there were no more than 5,000.
Goerges has also qualified to defend her title at the WTA Elite Trophy Final in Zhuhai, China, starting 30 October, and will be back to defend her singles title at the 2019 ASB Classic starting 31 December.
The win solidifies Bouchard's impressive play as of late.
Later that year, with 2015 Toronto triumph, she became first teenager to win an event at Premier 5 level or higher since Azarenka at 2009 Miami. She will meet Bencic for the first time.
Having strolled to the title a year ago, Goerges will be back looking for more success.
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