The Belgian held on for much of the 14-kilometre climb, and looked as if he could keep some of his 20-second lead after UAE Team Emirates' Tadej Pogačar went on the attack.
Van Aert, who had kept the gap within reach until the final three kilometres, finished ninth on the stage and dropped to second place overall. However, with a 35-second deficit to Pogačar with three stages still to come, he has not yet given up hope of winning the overall title.
The fact that a Classics rider and former cyclo-cross world champion held Grand Tour champions like Pogačar, Simon Yates (Team BikeExchange) - who finished second on the stage - and Nairo Quintana (Arkéa-Samsic) to less than a minute's gap on a mountain finale is a major milestone for the Belgian's first crack at the general classification of a WorldTour stage race.
"I can be proud of this achievement, I'm going to keep trying anyway," Van Aert said to Sporza.
Van Aert was at a disadvantage against teams like Ineos Grenadiers, who led into the base of the climb and, with an attack from Egan Bernal 7.5km from the line, succeeded in isolating Van Aert from his Jumbo-Visma teammates. While Van Aert closed down Bernal's attack, he was unable to match Pogačar when he went 2km later.
"I was hoping Ineos or UAE would control it longer. But the teammates were soon gone and then I saw it coming that they would break it up quickly," Van Aert said.
"That was not to my advantage. I didn't respond powerfully and found my own pace. Of course, that caused me to be looked at. I really had to rider eight kilometres on my limit. It was the best tactic for someone with my options and I am happy with my performance."
Van Aert managed to shut down counter-attacks from Mikel Landa (Bahrain Victorious), Quintana, Sergio Higuita (EF Education-Nippo) and João Almeida (Deceuninck-Quickstep) after Pogačar and then Yates went clear. His pacemaking was even strong enough to distance earlier attackers Geraint Thomas and Bernal.
"There was a lot of wind and that makes it easier to attack," Van Aert said. "If I wasn't wearing that blue jersey, I could have saved myself for the final but that was not the case. There were a few guys better and a lot of guys were challenging me. They could wait and see, I couldn't."
Van Aert has three chances to close the 35 second gap to Pogačar – Sunday's Classics-style stage to Castelfidardo, a likely sprint stage to Lido di Fermo on Monday, and the final 10km time trial in San Benedetto del Tronto. There are also some sprint time bonuses to be had along the way.
"It's a big gap," Van Aert admitted. "The next stages should be on me and I'm definitely not giving up yet. I think it makes sense to look up and seize the opportunities that arise. I'm going to keep trying anyway."
Van Aert said he wasn't disappointed to lose the jersey and acknowledged that it was logical that a rider like Pogačar would take the race lead on such a climb.
"In my dreams, of course, I would have won, but anyone can dream. I must be happy and proud of this achievement," he said.
"The logic is also respected. Pogačar, together with my teammate Primož Roglič, is the best all-rounder of the moment. It is only logical that he takes over my jersey."
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