Bertens missed out on a place in the last four in Moscow when she was beaten 6-4 2-6 6-1 by the unseeded Mladenovic.
The No.2 seed now requires a withdrawal to feature in the season-ending tournament in China after failing to climb above the eighth-placed Serena Williams in the standings.
Mladenovic mixed up her game with skill and intelligence and faces a semi-final showdown with Bencic next.
Bencic will move ahead of both Bertens and Williams to grab a Shenzhen spot if she records one more win after a 7-6 (8-6) 6-1 result against qualifier Kirsten Flipkens on Saturday (AEDT).
The No.3 seed, from Switzerland, who has never played in the WTA Finals, drew on her fighting spirit to come from 4-1 down in the opening set and edged a tie-break after saving two set points, then dominated the second set.
Anastasia Pavlyuchenkova and Karolina Muchova will contest the other semi-final after getting the better of Veronika Kudermetova and Ekaterina Alexandrova respectively.
Meanwhile, defending champion Julia Goerges rallied from a set down to beat Monica Puig 1-6 6-2 6-3 and reach the last four of the Luxembourg Open.
The German will take on Elena Rybakina, a 6-0 6-4 winner against Laura Siegemund, while Jelena Ostapenko and Anna Blinkova will also do battle for a place in the final.