Williams, attempting to claim her first WTA Tour title since returning from maternity leave, was a break down at 3-1 in the first set in Toronto on Sunday when she declared she was unable to continue.
The 23-time grand slam singles champion sobbed as she was embraced by the 19-year-old Andreescu, after succumbing to an injury setback just a fortnight before the US Open gets under way.
Andreescu becomes the first home winner of the WTA Premier tournament since Faye Urban beat Vicki Berner in an all-Canadian final 50 years ago.
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