The Belgian rider had one of his best days on a bike, but had to expend too many of his resources fighting fires, which included three flat tyres.
Lampaert, who finished third behind teammate Philippe Gilbert in the last edition in 2019, still managed a creditable fifth place on Sunday, but was left to rue what might have been.
"I don't say it quickly about myself, but I am almost certain that I was the strongest man in the race," he said, according to Belgian newspapers Het Nieuwsblad and Het Laatste Nieuws.
"If you puncture three times at crucial moments and you are still fifth, then I think you can say that you had very good legs."
Lampaert suffered his first puncture on the first of the 30 cobblestone sectors that had been rendered treacherous by the rain and the resulting mud. He flatted again on the Wallers sector inside the final 100km and then again at a crucial point of the race on the Tilloy to Sars-et-Rosières sector with 70km to go.
Mathieu van der Poel was making his race-shaping attack and Lampaert was initially alive to it, but had to relent when he felt the air draining out of his front tyre again.
"On the first cobblestone sector I already had a flat tyre and I had to return without cars. That was hell," he explained. "After that, I was perfectly in second position in Wallers, but I had another puncture. I got a wheel from the neutral car, but they weren't super wheels. I asked for one more from Alpecin-Fenix, but he wouldn't give one. Eventually I came to a soigneur of ours, but I had already lost a lot of time.
"When Van der Poel attacked, I was in the wheel, but I seemed to be leaking in the middle of the cobblestones. It's a shame, but it's part of Paris-Roubaix."
Lampaert then found himself in a chase group alongside Wout van Aert but, despite chasing less than a minute behind the group containing Van der Poel and eventual winner Sonny Colbrelli, they would never see the front of the race again.
Lampaert won the sprint from that group to take fifth place on the day.
"Frustrated? No, not that, but I am disappointed, yes," he said. "Three times flat while riding with such a pair of legs. Hopefully it will come again some day. The fact that my son was waiting here on the track ensures that you can immediately put everything into perspective."
The 2021 Paris-Roubaix was the first time in a decade that no Deceuninck-QuickStep rider finished on the podium. The Belgian squad, 13-time winners of the Hell of the North, came into the race with options aplenty but the bad luck didn't stop with Lampaert.
Tour of Flanders champion Kasper Asgreen suffered two punctures in quick succession in the Arenberg forest, just as the race was heating up, while Florian Sénéchal was left stranded after just 100km.
"I had a flat tire in the sector ahead of Wallers, and our car drove past me. I had to wait way too long to get back on track," he said.
Deceuninck-QuickStep team director Wilfried Peeters countered: "I never saw Florian. You don't recognize anyone with all those smeared faces, especially if they're standing between the soigneurs. He didn't say anything on the radio either."
The one rider who seemed to escape bad luck was Zdenek Stybar but, even after his promising top-10 at Worlds, the Czech rider didn't have the strength to bring home a result and finished 26th.
"I never had the legs to follow the best", he said. "I was hoping that I would get better as the miles went by, but I couldn't. Especially in the last 50 kilometers, I felt my body getting emptier and emptier."
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