Italian Vincenzo Nibali (Bahrain-Merida), sitting fourth overall in the 2018 Tour de France, is on his way to the Grenoble hospital with a suspected vertebra fracture.
He fell at 3.8 kilometres from the Alpe d'Huez finish when following Chris Froome. Sky team-mate and race leader Geraint Thomas squeezed by, riding over Nibali's wheel.
"The blow gave me big pain in my back, it blocked it," the 2014 Tour winner explained just after finishing the stage.
"Bardet was up the road after his attack, there were motors there, and they were squeezing in on the road, there was not space. There were two police motorbikes, I was following Froome, I was feeling good, then it slowed and I fell down."
Amazingly, Nibali returned to his bike and closed to 17 seconds. He finished the stage 13 seconds behind Alpe d'Huez winner Thomas and held his fourth spot overall, 2-37 minutes behind.
"I don't even know myself [how I am]," he continued before going to the X-ray truck at the top of the famous climb.
"I have a back that hurts, and until I see with the exams, I don't know. I took a hard blow, it was hard to breathe when that happen to start again right away."
Without an airlift option, an ambulance took Nibali for the 65-kilometre drive to the Grenoble hospital. A decision will be taken later if he can continue.
The Sicilian, winner of all three Grand Tours, appeared to be ready for the fight in the coming week and a half remaining. He attacked with 9.4 kilometres to race and once back in the favourites group, readied for more.
"I'm upset, my form was there, and today I believed in it strongly, my legs were going well and I felt good," Nibali said.
"The first attack I made was to see if someone was hurting or was going well, but the idea was to attack in the final again."
The incident is one of many in recent years. Today, fewer fans lined the 21 hairpins than in recent years, but space was still difficult for the peloton.
"I'm not sure what happened to Nibali, I think Geraint Thomas went over his back wheel and nearly came off himself," Team Sky boss David Brailsford said.
"I heard that Jakob Fuglsang nearly fell off. This is professional sport, and you expect professional athletes to play and entertain without being impacted on by the crowd.
"I know that this is part of the joy of our sport, how close the crowd gets, but we know that if impacts the race as it did today with Nibali, then that's too much."
Nibali continued: "These are things that can happen with so many fans and with the roads narrowing in on us. What else can I say?"
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