I think Paris-Roubaix has to be the toughest race because you have the terrain - there are anything between 52 and 55 kilometres of cobbles over the course of around 260 kilometres.
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.
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.
"I broke a finger recently and didn't think I'd make Paris-Roubaix because of that", he said.
He suffered cardiac arrest and was airlifted to hospital in Lille but died at 21:40 BST on Sunday. "I'm exhausted after this race but not as exhausted as in previous years when I didn't win it for some reasons", Sagan said.
The 23-year-old's death was confirmed by his team Veranda's Willems-Crelan in a statement on their Twitter account.
Spieth takes command in 1st round
After opening with a 73 - and boldly proclaiming that he was right in the mix of things - he struggled to a 3-over 75 Friday. He took advantage of his length and made birdie at 13 and then rolled in a 30-foot birdie putt at 14.
"No update on Michael Goolaerts yet", the team later said in a statement.
British Team Sky rider Geraint Thomas was forced to abandon the race after crashing on the first section of cobbles. "We kindly ask to refrain from speculation as we wait for an update on his situation".
"Our thoughts are with his family and friends now", the team said.
Swiss national champion Diller led Sagan into the velodrome but the latter used his superior kick to win only his second of cycling's five Monuments - two years after his triumph at the 2016 Tour of Flanders.
"Amazing. I'm so exhausted after this race".
The 257-kilometer race is also known as the "Hell of the North" because of its treacherous profile including 54.5 kilometers of cobblestones spread out over 29 sectors. "I've done my best", Sagan said.