Nairo Quintana (Movistar) rode clear of number one rival Simon Yates (Mitchelton-Scott) on the Camperona climb, but admits that he has to deal with a much wilder field if he wants to win the 2018 Vuelta a España.
Oscar Rodriguez (Euskadi-Murias) won the 13th stage from an escape group, but behind, 2016 Vuelta winner Quintana rode clear with Yates on the short and steep finishing climb.
"Un día duro," Quintana said of the hard day. "This is practically the first real mountain stage, and it was good to get time.
"There are many rivals, not just Yates. It's still open, anything can happen at any time and you cannot afford a mistake – we hope to keep going with the same sensations and keeping working well as a team – and keep the momentum we have."
The stage opened up a trio of summit finish stages ending the second week of racing. The Quintana/Yates duo rode clear, gaining 20 seconds and more on their direct rivals. But more hard stages remain on tap: Saturday's stage with a four-kilometre summit finish at Les Praeres and 11.7 kilometres to Lagos de Covadonga on Sunday.
Jesus Herrada (Cofidis) still leads the overall, but the favourites for the 2018 Vuelta sit just behind. Yates, in second overall, leads the virtual classification by eight seconds over Quintana, 12 seconds on his Movistar team-mate Alejandro Valverde, 41 seconds on Miguel Angel Lopez (Astana) and 51 seconds on Rigoberto Uran (EF Education First-Drapac).
"It was a day for the five favourites: Nairo, Yates, Rigo, Kelderman and Lopez, and they were all there: attacking, aggressive and beautiful cycling. Bueno," said Movistar sports director Pablo Lastras.
"Yates is a great rival, he's still showing what he can do, it's just a question of him getting to Madrid and showing himself at the right time [like what didn't happen in the Giro]. He's the number one rival for us."
Yates led the Giro d'Italia for 13 days this year, but cracked just two days to go as Chris Froome rode clear for the overall win.
He and the Mitchelton-Scotto team brought the lessons that they learned and experience here to the Spanish Grand Tour. The pressure, however, is on Colombian Quintana who rides for a Spanish team and has yet to win a Grand Tour since the 2016 Vuelta to underline his star status.
"We are good. Nairo is very well and I see him doing well in the next days," Lastras added.
"Pressure is on Nairo? Yes. He feels that he has to win this Vuelta. He's getting there day by day, showing himself and I think he'll do it. Sunday is going to be the stage for him to show that."
Yates rode down the three kilometres to reach the team bus alone after his bother and other team-mates arrived.
"How dangerous is Nairo? Very. We knew that before today and before we started the race," Yates said.
In the Giro d'Italia, Yates faced rivals who could easy rip time into him in the time trial: Froome and Tom Dumoulin. In Spain, Quintana poses a different threat, someone who can go head to head with him in the mountains but also suffers in the time trial.
"It doesn't really change [from Chris Froome or Tom Dumoulin]. It depends, I don't know yet. He was strong again like I knew he would be. I did a good round which I'm happy with. We have a few more hard days to come now on the weekend.
"It's not really mano a mano [in the mountains] because they have a very strong team. Valverde is always there or there abouts, obviously. I might be a bit outnumbered there, but we'll see. I don't know [how the weekend stages will play out]. If I have the same legs again that I had today then I'll be happy."
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