Sagan has a commanding lead in the points competition and only needs to finish in Paris on Sunday to earn his record-equalling sixth green jersey. He was descending off the second climb of the day on the Col de Val Louron-Azet when he overcooked a corner and, like Philippe Gilbert (Quick-Step Floors) the day before, went off the road and into the forest.
"I crashed in a turn - in the corner I made a mistake," Sagan told a scrum of reporters at the finish. "It looked like a fast corner, but after I just went a little more right - I was braking but it wasn't enough. After I flew through the forest and I hit a big rock with my ass."
Sagan remounted and finished the stage 134th, more than 26 minutes behind winner Nairo Quintana (Movistar). Asked if he would continue racing to Paris, Sagan left no doubt.
"Oh yeah," he said. "It would be very easy to just go home, but two or three days before Paris, no. It could be worse. I have some scratches and I hit the muscle on my ass. I hope it is going to be better in one or two days."
The team later confirmed that X-rays found no fractures. "Peter suffers from heavy abrasions and some bruises on his right side. A final decision about taking on stage 18 will be taken from the team tomorrow morning," the team reported.
The news is especially bad for Sagan considering Thursday's stage will finish in a likely bunch kick, and with a reduced sprint field because of abandons, the Slovakian would normally have a good chance of adding another stage win to his tally of three so far.
"We will see tomorrow morning how I feel," Sagan said when asked if he would contest Thursday's stage finish in Pau. "I just did my pace on the last climb, and I finished OK."