Demi Vollering held off two-time world road race champion Annemiek van Vleuten on an uphill finish to win stage five of La Vuelta Femenina.
The SD Worx rider, 26, beat her Dutch compatriot to the finish line by three seconds and now leads the overall standings by five seconds.
Overnight leader Marianne Vos slipped back on Puerto de Navafria, a category one climb, midway through the 129km stage from La Cabrera.
She finished the stage in 26th place.
Vollering was one of 24 riders to break clear on Puerto de Navafria.
On the final climb only she, 40-year-old Van Vleuten and Ricarda Bauernfeind remained in contention with two kilometres to go to the mountaintop finish in Mirador de Peñas Llanas.
Movistar rider Van Vleuten attempted an attack with 300 metres remaining only for Vollering to counter, overtake and power to the line.
Claire Steels, riding for Israel Premier Tech Roland, was the highest-placed British rider in 24th, finishing the stage two minutes behind Vollering.
Lizzie Deignan, who is continuing her return to racing following the birth of her second child, was 74th, almost nine minutes behind the winner.
Saturday’s stage six – 106.1km from Castro-Urdiales to Laredo – features two category two climbs on the race’s penultimate day.
Stage five results
1. Demi Vollering (Ned/Team SD Worx) 3hrs 33mins 25secs
2. Annemiek van Vleuten (Ned/Movistar Team Women) +3secs
3. Ricarda Bauernfeind (Ger/Canyon-Sram) +9secs
4. Evita Muzic (Fra/FDJ-Suez) +13secs
5. Gaia Realini (Ita/Trek-Segafredo) +27secs
6. Elise Chabbey (Sui/Canyon-Sram) +30secs
7. Riejanne Markus (Ned/Jumbo-Visma) same time
8. Juliette Labous (Fra/Team DSM)
9. Olivia Baril (Can/UAE Team ADQ)
10. Margarita Victo Garcia Canellas (Spa/LIV Racing Teqfind) +50secs
General classification
1. Demi Vollering (Ned/Team SD Worx) 13hrs 27mins 1sec
2. Annemiek van Vleuten (Ned/Movistar Team Women) +5secs
3. Riejanne Markus (Ned/Jumbo-Visma) +12secs
4. Elise Chabbey (Sui/Canyon-Sram) +27secs
5. Juliette Labous (Fra/Team DSM) +51secs
6. Marlen Reusser (Sui/Team SD Worx) +1min
7. Olivia Baril (Can/UAE Team ADQ) +1min 7secs
8. Katarzyna Niewiadoma (Pol/Canyon-Sram) +1min 27secs
9. Margarita Victo Garcia Canellas (Spa/LIV Racing Teqfind) +1min 28secs
10. Erica Magnaldi (Ita/UAE Team ADQ) same time