Bacsinszky, who is returning from a long term leg injury, upset the odds to record a 7-6 (2), 7-6 (5) win which ended Belarussian Sabalenka's chances of competing in the season-ending finals in Singapore.
Second seed Caroline Garcia made her way into the semifinals after Croatian opponent Petra Martic retired hurt. France's Garcia led 6-2, 1-0 when Martic was forced to pull out.
Garcia will play Su-Wei Hsieh in the semifinals. The No. 5 seed progressed ahead of Elise Mertens who retired with the score 6-2, 4-0.