The 26-year-old Slovakian, who won three stages over the last three weeks, will collect his fifth straight points title when Tour ends in Paris this evening. It follows a year that saw him win the world championship and in the rainbow jersey, the Tour of Flanders.
"He's unbelievable," said Bernie Eisel, who leads out Mark Cavendish in team Dimension Data and regularly sees Sagan in the classics.
"I don't care if he wins the green jersey another five times. He's definitely the best guy out there, he can win on every parcours.
"He's good for our sport because he attracts people from outside cycling. You like it or you don't. I think he's just cool. Other riders say they don't have time for such fun, but he just does it."
Sagan appears to enjoy having fun, from mock Grease and Rocky videos to bizarre wedding and fake haircut photographs.
As much as he laughs, he wins. Since becoming professional in 2010, Sagan rocketed through the ranks with classics victories, and stage wins and points jerseys in almost every stage race.
"It's how he raced the Tour de France, almost every day in the break," last year's Milan-San Remo and Paris-Roubaix winner, John Degenkolb (Giant-Alpecin) said in a quiet moment during the race.
"He's a much better climber than me. I see him... Or I maybe don't even see him, I just hear over the radio, 'Sagan again in the group.' I think, 'Dude, how's this possible?' It's definitely very impressive.
"He's definitely one of these big, big talents that we have every couple of years in cycling."
Sagan won so much so quickly that some began to ask if he could challenge for the Tour de France overall. Roberto Amadio, the general manager of Sagan's Cannondale team at the time in 2014, said that his talented start could start by aiming for the overall in Tirreno-Adriatico and Paris-Nice and then think about the Tour's yellow jersey.
"The Tour de France overall is too much, but he'll definitely be fighting for the green jersey, for stage victories, for monuments, for classics," Degenkolb said. "It could even possible that he wins all five monuments because he's a good climber. He could go for that."
"For sure, the classics of the north are best suited for him," said Etixx-Quick-Step sports director Tom Steels, who won Tour sprints and like Sagan, Ghent-Wevelgem twice. "San Remo, Flanders, Roubaix... All the Flemish classics, and the Tour de France. He'll do it many times over.
"To beat him for the green jersey is almost impossible. The yellow jersey? That's difficult. He's a really good rider, but to be there every day on the long and steep climbs... He's not made for that."
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