With three kilometres to go, the top two pair opened a 29-second gap to Tadej Pogacar (UAE Team Emirates), Miguel Angel Lopez (Astana) and Rafal Majka (Bora-Hansgrohe) as Nairo Quintana (Movistar) struggled 25 back.
“I had good legs, and so I tried to attack from almost the foot of the climb, to see what happened," Valverde said. "Everyone pretty much struggled up here, as well the two of us in the lead as the ones in pursuit.
"Should I have gained no advantage with that attack, you could argue the move was nonsense, but 40 seconds (to the group behind) is a nice gap."
By stage end, Roglic's team-mate Sepp Kuss had soloed to victory, ahead of Ruben Guerreiro (Katusha-Alpecin) and Tao Geoghegan Hart (Ineos).
Roglic and Valverde finished together, at two minutes 14 seconds while Lopez and Pogacar lost another 41 seconds to the overall leader and Quintana faded another 96 seconds away.
"I worked well with Roglic, we went ahead and opened a good distance with the other GC contenders," Valverde said. "Thanks to the team for giving me a chance to be here again contesting the Vuelta, and thanks to [Marc] Soler who did such a nice job.
"We did what we had to do, battling hard from the beginning and sending Soler into the breakaway. Marc really tried, yet once he saw the stage victory was becoming hard for him, he stopped to help me out."
So what's next for Valverde as he attempts to not just land another Vuelta podium but breach the Roglic and Jumbo-Visma defences?
"As during the whole race, we continue to go day-by-day," he said. "We all have been suffering a lot in this Vuelta, but we’re happy to be in this second place so far.
"It will be more of the same tomorrow; a very demanding stage, as always here in Asturias. A Vuelta win for ‘grandpa’?...Grandpa hasn’t said his final word!”