Geraint Thomas knows he faces a battle to keep up with his young team-mate Egan Bernal when the Tour de France heads for La Planche des Belles Filles on Thursday.

As Peter Sagan was celebrating his 12th career Tour stage victory in Colmar, attention was already turning to the first mountain stage of this year's race.

Seven categorised climbs pepper the 160.5km route from Mulhouse to La Planche des Belles Filles, dramatic enough in its first three appearances in the Tour but even tougher this year thanks to an extra kilometre in which gradients reach 24 percent and the surface is gravel.

Much has been made of the five seconds that Thomas conceded to Bernal with a momentary lapse in the finale of stage three to Epernay, but there could be more significant gaps come Thursday evening.

Thomas said: "(The final climb) is hard. I did it just before (the Tour de) Suisse. That sort of climb favours the punchy, pure climbers, Egan for one. Then in other teams guys like (Adam) Yatesy, Richie (Porte), (Nairo) Quintana. It suits those guys."

But what of Thomas? The defending champion's form is all but impossible to read given his lack of racing, his Tour de Suisse cut short by a stage four crash.

Mitchelton-Scott sporting director Matt White warned that Thomas will have "nowhere to hide", but Ineos team principal Sir Dave Brailsford believes his star man's form is better than others might expect.

"He's done a lot of mountain training, he hasn't been sitting on his arse, that's for sure," Brailsford said.

"I think the Giro (d'Italia) lasted something like 90 hours and the guys trained 118 (in that time).

"OK, you're racing, racing, racing but they train hard, these lads."

Losing time on the climb would not necessarily be terminal for Thomas, who is looking forward to time trial and the long, gradual ascents of the final week, but it would be another setback as he and Bernal vie for supremacy.

And those that believe in history will note that in the climb's three previous appearances in the Tour, the man in yellow at the top wears it in Paris – Sir Bradley Wiggins in 2012, Vincenzo Nibali in 2014 and Chris Froome in 2017.

Perhaps the new finale means that rule no longer applies.

Thomas has reconned the route where Bernal has not, but some of that information may already be out of date. News emerged on the eve of the stage that the surface has been worked on in recent days, with even teams in the dark as to what that meant precisely.

"No one is quite sure exactly what quality of tarmac or gravel we will have," Team Ineos sporting director Nico Portal said. "Probably (Wednesday) evening we will know from ASO by the latest statement."

The climb should suit Yates. The 26-year-old Lancastrian knows most of it well from his days racing for French amateur squad CC Etupes as a youngster but admitted he has never ventured up to the new finish line.

"It was probably about 50km away from the team house so I know it quite well," the Mitchelton-Scott rider said. "I don't know the new gravel bit but I reckon I know it well enough.

"It's a tough climb, quite steep. It's going to be the first big test. I think all the climbers are looking forward to feeling out where they are, where their rivals are, and in a way getting the race started."

With the mountains looming, the general classification riders kept their powder dry on the lumps of stage five.

Many predicted a breakaway but once a number of the quick men made it over the final climb, a sprint finish was on.

Yates' team-mate Matteo Trentin launched his move first but Bora-Hansgrohe's Sagan had the power to come around the Italian and hold off Jumbo-Visma's Wout Van Aert for his first win of this Tour, while Deceuninck-Quick Step's Julian Alaphilippe was 10th to retain yellow.

"Exquisite," the former world champion said when asked how it felt.

"I just have to ride with passion and the victory comes," he added.

"I have to say thanks to all my team-mates. They have done a great job and finally, we have the Tour de France victory that we were looking for."