The Camperona summit finish, the first of four in a row, shook up the overall classification, but the favourites insist the Vuelta a España is not over with two weeks to race.
The road in northwest Spain went upwards for the final 8.5 kilometres, with the last 3.1 kilometres rising rapidly at 20 to 25%. Sky's Chris Froome appeared to be in control, but Nairo Quintana (Movistar) countered his move and rode clear. Quintana put 25 seconds into Alberto Contador (Tinkoff), who passed Froome.
Froome finished 33 seconds back with Quintana's Movistar teammate Alejandro Valverde and Esteban Chaves (Orica-BikeExchange) at 57 seconds back.
Quintana came out the winner. He now leads the overall classification by 19 seconds over Valverde, 27 over Froome and 57 over Chaves. Contador sits in seventh at 1-39 minutes. However, there are several summit finishes to come in the remaining two weeks.
"There are a lot of hard mountains still ahead of us," said Quintana, who in the Tour de France placed twice second and third this year behind Froome.
"These next two days, there will be a lot of tension and it will be very difficult, but with the team I have, we will be able to defend ourselves well.
"We saw that Froome could not respond to the attacks. I believe I will be able to keep attacking, because I am feeling very strong."
Chris Froome speaks before the 2016 Vuelta a España
Froome appeared to be pacing himself as he did on stage three's steep finish up Mirador de Ézaro. He accelerated when they drew near the line to move ahead but out of the norm, he faded.
Sergey Lagutin (Katusha) won from an escape, but Quintana took advantage of Froome's sudden slip. Contador, too. Froome minimised the gap, though. Out of the stars, only Chaves seemed to be on an off day. Chaves, second in the Giro d'Italia this May, crossed the line with team-mate Simon Yates.
"Well, I did lose a little bit of time today, but I'm still in there," Froome said before getting on his bicycle to warm down outside of Sky's bus. "[Quintana] showed he's in great shape. I think Movistar is going to be the team to go up against and this race.
"I wouldn't really write off [Chaves] from today because it was a special sort of finish with less than 10 minutes for that final steep part. So you can't read too much into it. But at the same time, it does show a little bit about where everyone is at and certainly the GC is taking shape now."
Froome explained that he is not in the same form that he was when he raced to his third Tour de France title in July. After the Tour, Quintana rested while Froome raced RideLondon and the Olympics, where he took the bronze medal in the time trial.
"It's still early days in the race and there's still a lot to fight for," Froome said.
"All in all, I'm still pretty happy with where I'm at. There's still a lot of racing to come like the time trial on stage 18 which should suit me very well. Apart from that, it's for Movistar to control the race now."
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