The three-time world champion survived to save his green jersey two days after crashing off the side of the road on stage 17.
"On the bike, I think we can say this was a very hard day," Sagan said. "The hardest one."
He began suffering right away in the 200.5-kilometre stage and began drifting back while the television cameras focused on Geraint Thomas (Team Sky) trying to defending his yellow jersey.
Primoz Roglic (LottoNL-Jumbo) escaped for the stage win and Thomas succeeded in his defence in Laruns, at the base of the Pyrénées. Sagan finished 38-23 behind in the gruppetto.
"On the bike it could be [the hardest day]. After the injury and crash, it's pretty bad to have to go on the bike. Yeah. It was bad [today]," Sagan said.
"Mentally, I'm good, I think, because there was no excuse to throw it away today. In my head, I thought 'I have to finish the race'. But physically it was really hard."
Sagan's team-mates Daniel Oss, Maciej Bodnar and Lukas Pöstlberger stayed at his side with the group also including sprinter Arnaud Démare and his Groupama-FDJ team-mates.
"I was surprised with the first climb, I thought, 'Hell, what am I going to do?' After two kilometres easy, I was almost dropped," he explained.
"I managed it well with the gruppetto, with my team-mates and the team-mates of Démare. My team-mates kept me mentally strong. They kept me in the race," Sagan said.
"I said 'come on, we have to make it.' [The time cut] is going to 50 minutes, and in the end it was 46 [minutes]."
The race has seen many sprinters call it quits in the mountains with other riders like Philippe Gilbert (Quick-Step Floors) saying it was too much after a crash.
Sagan's extra incentive came from the green jersey. He has the points classification mathematically won already, he just needs to finish in Paris to take his sixth points jersey.
Finishing stage 19 seemed like another win to add to the green jersey and the three stage wins that Sagan already has from this years race.
"Yes, this is much more than those three victories before for me. I need to survive tomorrow also, I hope it's going to be easier for me," added Sagan.
"If I was at any other race other than the Tour de France then I wouldn't ride, but I'm here and I have to finish. It's like waking up in the morning and going to school. I was also suffering when I was going to school. I think this is better."
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