Geraint Thomas believes the destination of the Tour de France yellow jersey could be decided by Wednesday, when a short but intense stage 17 hosts the final mountaintop finish of the race.
Welshman Thomas takes a 1min 39sec lead over Sky team-mate and four-time champion Chris Froome into the first of four days in the Pyrenees Tuesday when the race resumes following a rest day.
Dutch rival Tom Dumoulin is third overall at 1:50, Primoz Roglic fourth at 2:38 and Frenchman Romain Bardet fifth at 3:21.
Despite three tough days in the mountains up ahead, only one of them -- stage 17 -- ends on a mountaintop.
As opposed to downhill finishes, the dramatic summit finishes on the Tour are where Sky's rivals can realistically take back lost time.
And Thomas said: "It's the last mountaintop (finish) that will be most decisive."
Like most teams, Sky have previewed the routes of all three Pyreneean stages, as well as the penultimate-stage time trial from Saint-Pee-Sur-Nivelle to Espelette.
"We've seen the finals of them all," added Thomas, whose bid to become the first Welsh champion of the Tour could be tempered by Froome's ambitions for a record-equalling fifth yellow jersey.
Although 218 km long and with four categorised climbs, none of the ascents on Tuesday's 16th stage are over nine kilometres long.
And with a downhill finish, it is likely to entice the dozens of riders who have yet to join a breakaway, let alone win a stage.
But it, as with most long mountain stages, will be tightly controlled by Sky.
"It's a tough little finish tomorrow into Bagneres-De-Luchon," Thomas said.
The next day, however, all hell could break loose.
Stage 17 is only 65 km long but starts with the 14.9 km ascension of the Col de Peyragudes, is followed by the 7.4 km climb over the Col de Val Louron-Azet before the gruelling, 16 km climb to Saint-Lary-Soulan.
Adding spice is the fact the organisers, with a nod to Formula One, will line up the top 20 riders on a grid, according to their overall times.
"I think it's unique, it's going to be tough for sure," said Thomas.
"We expect a lot of attacks from the start. It's a big, big day. Probably one of the hardest climbs, the last climb.
"It finishes at high altitude. You can't get too carried away at the start for sure."