Bradley Wiggins' hopes of winning the Vuelta a Espana suffered a setback on Sunday as he lost the race leader's red jersey to Spaniard Juan Jose Cobo.
Briton Wiggins led Cobo by 55 seconds at the start of the 15th stage, but he struggled on the notoriously tough climb to the summit finish at Angliru.
Cobo broke clear on the final climb to win and earn a 20-second time bonus.
Britain's Chris Froome remained second in the standings, 20 seconds back, with Wiggins third, 46 seconds down.
After the stage Froome said: "The plan for today was to try and keep Bradley right up there going into that final climb.
"With those gradients it was basically a time trial from the bottom to the top and both of us were poised in really good positions on the road.
"We gave all we had all the way up there but today Cobo proved to be stronger than both of us.
"We've lost the jersey, which never leaves you with a nice feeling, but there's still a week of racing to go and we'll do everything in our power to keep battling and keep right up there in the standings.
"This second rest day couldn't come at a better time for me, just like the first one couldn't, because I'm absolutely shattered and tomorrow will be a welcome time to take things easy and recharge the batteries."
With just six stages remaining until the race ends on 11 September in Madrid, Cobo is now favourite for the overall victory.
There was little sign of the drama to come as the stage began at a relatively gentle pace.
A three-man breakaway was allowed to go out to five minutes but once the peloton had cleared the category two Alto de Tenebredo climb and the category one climb of Alto del Cordalm the leaders were reeled in.
And it was on the final climb of the day, the brutal Alto de l'Angliru, that Cobo made his bid for glory.
The climb peaks at a gradient of 23.5% which reduced many riders to walking pace.
But Cobo made a lung-busting dash for the summit, breaking his nearest challengers on the stage and, more crucially, Wiggins and Froome who were in the pack and visibly struggling.
Cobo crossed the line in four hours, one minute, 56 seconds, a gap of 48 seconds over Froome, who finished alongside Wouter Poels of the Netherlands and Russia's Denis Menchov.
Triple Olympic champion Wiggins, who was fifth, 1:21 behind Cobo, said on his Twitter page: "Well Cobo was just too strong today, congrats to him. Thought my race was over with mechanical on the last descent but managed to come back."
Monday is a rest day before the Vuelta heads toward the Basque country for the first time in 33 years.
Results from the 15th stage of the Spanish Vuelta, a 89.5-mile (144 kilometre) mountain course from Aviles to Alto de l'Angliru:
1. Juan Jose Cobo, Spain, Geox, 4 hours, 01 minute, 56 seconds
2. Wouter Poels, Netherlands, Vacansoleil, +48 seconds
3. Denis Menchov, Russia, Geox, same time
4. Christopher Froome, Britain, Sky, same time
5. Bradley Wiggins, Britain, Sky, +1:21
6. Igor Anton, Spain, Euskaltel, same time
7. Joaquim Rodriguez, Spain, Katusha, +1:35
8. Maxime Monfort, Belgium, Leopard, same time
9. Bauke Mollema, Netherlands, Rabobank, same time
10. Sergey Lagutin, Uzbekistan, Vacansoleil, same time
Overall Standings (After 15 of 21 stages):
1. Juan Jose Cobo, Spain, Geox, 59 hours, 57 minutes, 16 seconds
2. Christopher Froome, Britain, Sky, +20 seconds
3. Bradley Wiggins, Britain, Sky, +46"
4. Bauke Mollema, Netherlands, Rabobank, +1:36
5. Maxime Monfort, Belgium, Leopard Trek, +2:37
6. Denis Menchov, Russia, Geox, +3:01
7. Jakob Fuglsang, Denmark, Leopard Trek, +3:06
8. Vicenzo Nibali, Italy, Liquigas-Cannondale, +3:27
9. Jurgen Van Den Broeck, Belgium, Omega Pharma Lotto, +3:58
10. Wouter Poels, Netherlands, Vacansoleil, +4:13