The queen stage of this year's Tour it features a hellish profile for the peloton, 200.5km between Lourdes and Laruns, that takes in vertical slogs up Col d'Aspin, Col du Tourmalet and Col d'Aubisque - aptly nicknamed the "Circle of Death".
"We're expecting the worst and hoping for the best", Thomas said. "We think there will be attacks straight from the start, it's the last mountain stage and the guys will be wanting to take any opportunity they can".
The Manxman is one of two Brits selected for the African team, alongside fit-again Scott Thwaites.
Team Sky told reporters that there was a misunderstanding when police asked the rider to stop, which caused him to crash.
"Obviously, I didn't want him to have a bad day, like he did", Thomas said of his teammate.
"I think tomorrow will be much harder, but I consider myself lucky that, after such a crash, I'm still in the race and not sitting at home".
"You can't get carried away at the start because the last climb is one of the hardest of the tour".
"But we have to keep on fighting".
A sprint was always on the cards but the battle for position was wide open given that the field of quick men has been decimated in this race, with Mark Cavendish, Marcel Kittel, Andre Greipel, Dylan Groenewegen, Fernando Gaviria and Michael Matthews all having withdrawn at various points.
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The question of who was Sky's number 1 was answered in those short metres as Thomas could follow the accelerations whilst Chris Froome didn't have the legs to go with the trio, with Egan Bernal dropping back to help him.
Greipel, one of several sprinters to pull out in the Alps, while others, like Mark Cavendish and Marcel Kittel, missed the time cut, accused Demare of holding on to a auto to make it through the mountains on stage 17.
It was not clear that apology had been accepted as Demare said he had used Greipel's "insult" as motivation.
"I'm delighted", said Alaphilippe. "Obviously that insulted me. It gave me confidence, because if Froomey is suffering then everyone is suffering and I was feeling good", said the 32-year-old Welshman.
"I was thinking about Greipel", he added.
The Bora-Hansgrohe rider hobbled on to his bike in the morning but had put his team on the front in the final few kilometres before eventually finishing eighth. "This is now the fourth grand tour that I've raced consecutively and I'm looking forward to a bit of rest after this".
Pau: Frenchman Arnaud Demare flirted with the rule book on his way to holding off sprint rival Christophe Laporte and claim a well-deserved victory in the 18th stage of the Tour de France on Thursday.
Dumoulin is the world time trial champion and could gain plenty of time there.
After the drama of the past two days in the mountains this was a largely uneventful stage, no doubt welcome to those still with an eye on the fight for yellow.
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