The 21-year-old Kiwi was in the front group and appeared to be handling the tricky cobbles of northern France with aplomb next to team-mate Stefan Kueng.
Pithie lost contact when he crashed on a corner with 33km left to race.
He had been in the five-rider group chasing eventual winner Mathieu van der Poel but suddenly lost any chance of a podium spot, finishing seventh, 3min 48sec behind the winner.
"I can look back at my race and also be happy with seventh, but I could have been playing for the podium", Pithie told media, his face covered in mud and blood.
“My head got big, I made a mistake, and I crashed, taking me out of that group.
“I just went too fast into a gravel corner. I misjudged it and wiped myself out."
Van der Poel stormed to a second victory in the Paris Roubaix after the Dutchman launched a stunning solo attack 60km from the finish line to win his second Monument of the year following his Tour of Flanders victory the previous weekend.
On the 260km "Hell of the North" route which had 29 cobblestone sectors, Van der Poel’s victory by 3min was the largest winning margin in 22 years as he rode to his sixth Monuments victory.
As he crossed the line with his arms outstretched, soaking in the crowd’s applause, his time of 5hr 25min 58sec was recorded as the quickest Paris Roubaix in history.
Van der Poel became only the 10th rider to win both cobbled Monuments — the Tour of Flanders and Paris Roubaix — in the same season and only the second rider to do it with the rainbow jersey after Rik van Looy in 1962.
"I wanted to make the race hard from there [60km out] on because I know that’s my strength. I felt super good today and when I had the gap, I knew [I could win] with a tailwind to the finish line for the most part," Van der Poel said.
"Of course, you always know in Roubaix that there’s the possibility of a puncture. But I had the team car with me and I was quite confident.
"I could really enjoy the moment more than last week [at Flanders] because then I was on my limit. This time, I could really enjoy the last part."
Van der Poel launched his solo attack on sector 13 and quickly built up a huge gap.
Mads Pedersen and Jasper Philipsen were among those who gave pursuit but the pack failed to organise quickly, looking to each other to seize the initiative as Van der Poel disappeared into the distance.
Van der Poel also had his Alpecin-Deceuninck team mates behind him to thank as they disrupted the chasing pack and looked to deny riders any breakaway attempts.
As Van der Poel rode towards victory, Philipsen decided to accelerate with 14 km to go to seal second place while Pedersen and Nils Politt joined him in a three-way battle for the last two podium spots.
When they entered the Roubaix Velodrome, UAE Team Emirates’ Politt went for it early but it was Philipsen who finished second for an Alpecin-Deceuninck one-two while Pedersen was third for Lidl-Trek.
— Staff reporter with additional reporting by Reuters
