Zdenek Stybar (QuickStep Floors) was next to try a notable attack, by which point only Dillier, Jelle Wallays (Lotto-Soudal) and Sven Erik Bystrom (UAE Team Emirates) remained in the breakaway.
When Stybar was brought back, two accelerations from Van Avermaet – now back in the bunch – strung out the peloton but it was Sagan who seized the initiative with little more than 50km to race.
With the chase disjointed, his advantage quickly opened out and he picked off the escapees – leading through the Carrefour de l’Arbre sector with Dillier on his wheel.
Dillier led into the Velodrome, and took the high banking, but when Sagan opened out his sprint he was not going to be denied.
Tour of Flanders winner Niki Terpstra (QuickStep Floors) took third place, finishing just less than a minute later, while Van Avermaet led home what remained of the counter-attacking group at 1’34” in fourth place.
Paris-Roubaix 2018: result
1) Peter Sagan (SVK) – Bora-hansgrohe – 5.54.06hrs
2) Silvan Dillier (SUI) – Ag2r-La Mondiale – ST
3) Niki Terpstra (NED) – QuickStep Floors +57”
4) Greg van Avermaet (BEL) – BMC Racing +1.34
5) Jasper Stuyven (BEL) – Trek-Segafredo – ST
6) Sep Vanmarcke (BEL) – Team EF-Drapac
7) Nils Politt (GER) – Katusha-Alpecin +2.31
8) Taylor Phinney (USA) – Team EF-Drapac – ST
9) Zdenek Stybar (CZE) – QuickStep Floors
10) Jens Debusschere (BEL) – Lotto-Soudal