The Tinkoff-Saxo man profited from team Giant-Alpecin's work. He came alongside German John Degenkolb (Giant-Alpecin) and held off Frenchman Nacer Bouhanni (Cofidis). The win was his first in a Grand Tour since the 2013 Tour de France when he raced for Cannondale.
At the Tour de France last month, he won the green points jersey for the fourth straight time but without a single victory and placed second five times and third twice.
The trend looked to continue when he and Tinkoff placed second in the opening team time trial of the Vuelta on Saturday.
"It's funny also. All those seconds are almost impossible. You have to be one time first or third, no!?" Sagan said.
"That's sport, one time up or down. I'm stable...I’m always in the first five! I'm happy for the win, I was looking for it the whole Tour de France. Now it's come."
Sagan's second places and near misses have not been so funny at times. At the Classics this year, he was due to lead Tinkoff after joining over the winter on a reported €4m contract.
Despite the promise that came with second in the 2013 Milan-San Remo and the Tour of Flanders when the Slovak was only 23-years-old, he could only take fourth this season in those two races and lower down in others.
Earlier this year, team owner Oleg Tinkov said that he wanted to try to reduce the value of his contact after poor form. He told La Gazzetta dello Sport newspaper, "You don't sign a three-year deal and then don't bring in results."
"You have to ask him [if this win relieves the pressure]. I don't have too much pressure in the team, I just want to win," Sagan said. "Also, Oleg understands it's sport and he knows I'm doing my best."
Sagan explained that the Vuelta serves a greater purpose. He is in Spain fighting for stage wins, but also for condition to be ready to try to win the world championship road race in Richmond, Virginia, next month.
A win at the worlds, and not second or third, would quiet most of his critics.
"I come here mostly for preparation, to do some race kilometres, then I'll see how it's going," added Sagan. "I'm looking forward to the world championships."
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