The 25-year-old Errani, returning to Paris less than a year after reaching the French Open final, was in command when Bertens was forced out with back pain.
The world number seven had already seen Madgalena Rybarikova of Slovakia retire in the first set of their second-round encounter but dismissed the idea she had enjoyed an easy path to the final.
"Oh no, we can't say that. I admit it does not happen very often but I played more than three hours last night," Errani said referring to her 7-5 4-6 7-5 victory over Spaniard Carla Suarez Navarro in the quarter-finals.
Errani will bid for a seventh career title on Sunday against 45th-ranked Mona Barthel of Germany who only needed 65 minutes to sweep aside 19-year-old Kristina Mladenovic of France 6-1 6-4.
Mladenovic had thrashed second seed Petra Kvitova 6-3 6-4 a day earlier.