Michael Goolaerts has died at the age of 23 after an incident during the Paris-Roubaix classic.
He has been in superb form this season - placing ninth at Dwars dooor Vlaanderern and 20th at both Driedaagse De Panne and Kuuren-Brussel-Kuurne.
Dillier's classy ride was enough to seal second, with former victor Niki Terpstra (Quick-Step Floors) completing the podium.
Sagan thus became the first rider since Bernard Hinault in 1981 to win the Queen of the Classics in the rainbow jersey of a world champion.
"It's unbelievable to win Paris-Roubaix". With 55km left, Belgium's defending champion Greg van Avermaet and Sagan attacked one after the other and the Slovakian quickly caught the three breakaway riders. "I have to say that I was not involved in any crash or puncture or any kind of mechanical this time, so I could save some energy for the finale".
Sagan had attacked from a group of favourites with 54km to go in the race in France on Sunday.
Australian PM dismisses reports of Chinese military base talks with Vanuatu
The Sydney Morning Herald said China had approached the Pacific nation about the possibility. She didn't have more say, as she had only seen media reports, she said.
Team Sky said in a statement: "Everyone at Team Sky is heartbroken by the loss of Michael Goolaerts".
It is his second win in the "monuments" - cycling's five most prestigious one-day races - following his Tour of Flanders victory in 2016. The rider was found unconscious by race doctors.
Unfortunately, 22-year-old Belgian cyclist Michael Goolaerts fell victim to these incredibly hard circumstances during this afternoon's race. I said before the race that my goal was to do my best.
The commissaire's report says that the riders took the refuelling from team staff on foot, and so Sagan's Bora-Hansgrohe and Dillier's Ag2r La Mondiale teams have also been fined 1000 CHF each.
Goolaerts rode in support of cyclo-cross world champion Wout van Aert of Belgium in the race dubbed the "Hell of the North" because of the hard terrain it covers.