Hopes were high for the Spaniard, who is a strong time triallist on his day and finished ninth in the recent Tour de France time trial, which, like the Vuelta stage, paired 30km of flat with a steep final climb.
However, Mas could only manage 16th place on the day and was bettered by all four riders who started above him in the overall standings: Primoz Roglic, Richard Carapaz, Hugh Carthy, and Dan Martin.
Mas was losing time as early as the first checkpoint on the flat, and failed to make up for it once he'd switched to his road bike for the sharp 1.8km final climb. He shipped 1:43 to stage winner Roglic, 1:18 to Carthy, 54 seconds to Carapaz, and even 23 seconds to Martin, who is far from a time trial specialist.
"For me, today has been a disaster. I didn't find my rhythm, I didn't find a good position on the TT bike," Mas said on Spanish TV after the stage.
"That's part of this sport. There are good days and bad days. It wasn't the best day for me, but it was a day to learn - a lesson for the future that will serve me well in the coming years."
Both Mas and his team director Pablo Lastras defended the decision to change to a road bike not at the foot of the climb, like all the other riders, but on the climb itself, with the change coming from the team car on the first bend.
"Each little bit of time is important," said Lastras, who said they were "trying to get the most out of his momentum. As for question marks over the legality of the change, he insisted: "The rules say you can do it, so all legal."
Mas is still fifth overall, with a comfortable three-minute buffer over Wout Poels in sixth, but he is now much further from the red jersey, and even the podium would seem a tall order with five stages to go. He's 3:23 down on stage winner and overall leader Roglic, 2:44 down on Carapaz in second, 2:36 down on Carthy in the final podium position, and 1:41 off Martin in fourth.
The race continues this week with a series of medium mountain stages before Saturday's summit finish at La Covatilla provides the climax to the fight for the overall title.
"From tomorrow we'll speak among ourselves about how to go about the next stages, and how to approach the final week of the Vuelta and try to keep fighting for that podium," Mas said.
Lastras, meanwhile, claimed others too had suffered a bad day and hinted at teams uniting to usurp Roglic and his Jumbo-Visma teammates.
"It was a bad day for us but also for other teams, so I hope there's a good alliance so that we have some spectacle in what's left of the race."
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