Vollering won the 171.9-kilometre stage from Sisteron to Nice after attacking on the final climb, about 6.5 km from the finish.
Niewiadoma-Phinney and Italy's Elisa Longo Borghini initially managed to stay with her, but both eventually fell behind as Vollering finished alone, 17 seconds ahead of her rivals.
Niewiadoma-Phinney, who is seeking her second Tour de France overall title after winning in 2024, is 8 seconds behind Vollering heading into Sunday's final stage.
The Polish rider criticised Vollering’s French teammate, Céline Gery, after the finish, accusing her of deliberately blocking her near the barriers at the start of the final climb.
"I was super annoyed at the beginning of the climb because Gery, on purpose, just blocked me to the barriers, where I had to stop pedalling," Niewiadoma-Phinney told reporters. "I feel like if they want to compete, they should compete [with] fair play, and not look to close me into the barriers, because I feel like it’s so childish. Honestly, I lost all the respect for them for that."
Another Polish rider, Dominika Włodarczyk, finished sixth on Saturday, 38 seconds behind Vollering. She is eighth overall, 5 minutes and 46 seconds behind the leader.
The final stage, starting and finishing in Nice, will be held on Sunday.
Niewiadoma-Phinney won the Tour de France Femmes in 2024. She finished third in 2022, 2023 and 2025.
(gs)
Source: IAR, PAP, Eurosport, sports.yahoo.com