Defending champion Demi Vollering won stage one of the Itzulia Women with a dominant attack on the final climb.
The 26-year-old Dutchwoman, who rides for SD Worx, broke clear 12km from the finish of the 122km stage from Etxebarria to Markina-Xemein in Spain.
Swiss team-mate Marlen Reusser was second, 47 seconds behind, with Poland’s Kasia Niewiadoma of Canyon-Sram two seconds further back in third.
World champion Annemiek van Vleuten, riding for Movistar, finished fourth.
“I saw it was a really nice climb where I could gain time,” Vollering said.
“My goal was to go as early as possible and gain a lot of time. We did really good as a team. They kept me safe by staying at the front and going into the downhills first.
“They also did the lead-out perfect up to the bottom of the climb with Marlen putting in a good pace.”
The 133km second stage runs from Vitoria-Gasteiz to Amurrio on Sunday, with the final 115km stage in Donostia on Monday.
Stage one result
1. Demi Vollering (Ned/SD Worx) 3hrs 16mins 32secs
2. Marlen Reusser (Sui/SD Worx) +47secs
3. Kasia Niewiadoma (Pol/Canyon-Sram) +49secs
4. Annemiek van Vleuten (Ned/Movistar) +51secs
5. Olivia Baril (Can/Team ADQ) Same time
6. Evita Muzic (Fra/FDJ-SUEZ) +1min 25secs
7. Liane Lippert (Ger/Movistar) Same time
8. Soraya Paladin (Ita/Canyon-Sram) +1min 35secs
9. Loes Adegeest (Ned/FDJ-SUEZ) Same time
10. Eva van Agt (Ned/ Jumbo-Visma) Same time
General classification
1. Demi Vollering (Ned/SD Worx) 3hrs 16mins 22secs
2. Marlen Reusser (Sui/SD Worx) +51secs
3. Kasia Niewiadoma (Pol/Canyon-Sram) +55secs
4. Annemiek van Vleuten (Ned/Movistar) +1min 1sec
5. Olivia Baril (Can/Team ADQ) Same time
6. Evita Muzic (Fra/FDJ-SUEZ) +1min 35secs
7. Liane Lippert (Ger/Movistar) Same time
8. Soraya Paladin (Ita/Canyon-Sram) +1min 45secs
9. Loes Adegeest (Ned/FDJ-SUEZ) Same time
10. Eva van Agt (Ned/ Jumbo-Visma) Same time