Possible storms are forecast from 1400 local time (1200 GMT) when play is due to start, and rain showers from 1700 (1500 GMT) on a windy day in Paris.
Mladenovic, the 13th seed tipped as a potential Roland Garros champion after final appearances in Stuttgart and Madrid, will be backed by a hollering Court Philippe Chatrier crowd when she faces Bacsinszky, a semi-finalist here in 2015.
Bacsinszky beat Mladenovic in three tight sets in their last meeting, a Fed Cup tie in February.
Also on Chatrier, nine-times champion Rafa Nadal faces fellow Spaniard Pablo Carreno Busta, who has never beaten the claycourt master.
Nadal has been in ominous form, dropping only 20 games on his way to the last eight.
Djokovic, however, has been struggling for consistency and the 23-year-old Thiem is expected to provide a tough challenge even if the Austrian has won only one set in their five previous encounters.
The youngest player left in the draw, 19-year-old Latvian Jelena Ostapenko, faces 11th seed Caroline Wozniacki, who has reached the Paris quarter-finals for the first time since 2010.
(Reporting by Julien Pretot,; Editing by Pritha Sarkar)
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