Geraint Thomas delivered a major statement of intent in the defence of his Tour de France crown as he dropped Ineos team-mate Egan Bernal on the final brutal slopes of La Planche des Belles Filles.
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As Dylan Teuns won stage six from the breakaway and young Italian Giulio Ciccone snatched the yellow jersey off the shoulders of Julian Alaphilippe, it was the sight of Thomas defying his own predictions and powering clear of the main contenders at the last which caught the eye.
Much had been made of the five seconds the 22-year-old Bernal picked up due to a momentary lapse from Thomas in the finale of stage three in Epernay, and the gap between the team's co-leaders was only expected to grow here as gradients hit 24 per cent and the surface turned to gravel.
Thomas spent Wednesday explaining why the stage did not suit him and pointing to Bernal as one of the favourites to profit, but the wily Welshman perhaps knew more than he was letting on.
"I was feeling good but I was unsure," Thomas said. "I thought the steep climbs weren't my cup of tea. I was expecting others - (Nairo) Quintana, Egan, (Adam) Yates - would jump up there. It was a decent day in the end.
"It is one of those climbs where you have to patient. When Alaphilippe went clear at 800 (metres to go), quite early, I had the confidence to let him go and ride my own tempo and drive it all the way to the line from 350. I was starting to blow though. It is decent."
Decent would be an understatement for the day as a whole as it delivered the drama organisers had promised over seven categorised climbs and more than 4,000m of ascent.
Bahrain-Merida's Teuns and Trek-Segafredo's Ciccone were the last survivors of a 14-man breakaway on the 160.5km stage from Mulhouse, and both received rich rewards at the top of a climb which left many riders struggling to stand at the summit.
Teuns could celebrate a first career Tour victory while Ciccone - a break-out star of the Giro d'Italia in May as he took a stage win and the mountains classification - did just enough to take the yellow jersey by six seconds.
"It is unbelievable," the Trek-Segafredo rider said. "This feeling is crazy. I am 24, I here for the experience and now I have the yellow jersey."
Deceuninck-Quick Step's Alaphilippe did not give up the jersey without a fight. Having stuck with the group of favourites all day, he attacked as the road turned to dust near the summit.
At first no one reacted, but Thomas then found the reserves he needed to spring past the Frenchman, who slumped on to the barriers as soon as he crossed the line.
The seconds Thomas has gained may not be massive - two on Thibaut Pinot, seven on Quintana and nine on a group including Jakob Fuglsang, Richie Porte and Bernal - but this was a clear answer to questions over his form.
Mitchelton-Scott's Adam Yates, a former resident of these parts, seemed to be caught out by the new finish as he finished 14 seconds behind Thomas, alongside UAE Team Emirates' Dan Martin.
"I feel really good," Martin said. "I lost a bit of time on the dirt section but I'm not worried. I think this Tour will be about who has the least bad day and today was definitely not a bad day for me."
It was, however, a bad day for French hope Romain Bardet, who faded on the steep finale and had the final indignity of dropping his chain on the line as he shipped almost three minutes. All French hopes have immediately shifted to the seventh-placed Pinot.
Thomas, sitting fifth, is now the best placed of the main contenders, four seconds ahead of Bernal in sixth and nine up on Pinot. The likes of Steven Kruijswijk, Rigoberto Uran, Jakob Fuglsang and Yates lurk, no more than 35 seconds back, but then more serious gaps open up.
In the Tour's previous three visits to this mountain the man in yellow at the summit would be wearing it in Paris come the final stage. All the signs on Thursday suggest this edition has more twists to come.
Tour de France: Stage-by-stage guide
July 11, Stage 6: Mulhouse to La Planche des Belles Filles (160.5km)
Here we go then. The GC battle should come alive over seven categorised climbs, four of them category one, including the summit finish at La Planche des Belles Filles. Recent Tour history suggests whoever wears yellow here will wear it in Paris - see Bradley Wiggins in 2012, Vincenzo Nibali in 2014 and Chris Froome in 2017 - but the Champs-Elysees is long, long way away. This year sees a change at the summit, with an extra kilometre including 24 per cent gradients and gravel sections.
July 12, Stage 7: Belfort to Chalon-sur-Saone (230km)
After those exertions there is a recovery day of sorts with flat roads taking the race south west towards wine country. However, at 230km this is hardly a rest. Expect a long, slow day with every chance of a headwind harassing the peloton. Chalon-sur-Saone is where Britain's Brian Robinson won the second of his two Tour stages in 1959 - and by a margin of some 20 minutes. That will not happen this time though, with a sprint finish expected.
July 13, Stage 8: Macon to St Etienne (200km)
The Massif Central does not boast the beasts of the Alps or Pyrenees, but with seven categorised climbs the Tour's route designer Thierry Gouvenou has made sure this will be a draining day on the legs. A day with multiple possible attack points is designed to break the control of the big teams - read Team Ineos.
July 14, Stage 9: St Etienne to Briourde (170.5km)
A stage that finishes in Romain Bardet's home town is not really difficult enough for the great French hope to take victory, but how the locals would love to see the 28-year-old end Bastille Day in yellow with his family watching on. As for the stage, expect a big breakaway, full of Frenchmen, attack each other on the final climb which peaks 13 kilometres from the line.
July 15, Stage 10: Saint-Flour to Albi (217.5km)
The peloton must wait until Tuesday for their first rest day but as they climb on their bikes one more time, riders will be grateful most of the second half of this stage is essentially downhill. The sprint teams will hope for a straightforward day.
July 17, Stage 11: Albi to Toulouse (167km)
The last time a Tour stage finished in Toulouse, back in 2008, a young debutant by the name of Mark Cavendish took victory. After two tough years of illness, injury and misfortune the Manxman could certainly use a repeat 11 years on. With the mountains looming, it will be the sprinters' last chance for a while.
July 18, Stage 12: Toulouse to Bagneres-de-Bigorre (209.5km)
And so to the Pyrenees, though stage 12 is just a warm-up. Two category one climbs, the Peyresourde and the Hourquette d'Ancizan, provide the focus before a long descent into Bagneres-de-Bigorre where Ireland's Dan Martin took his first stage win in 2013.
July 19, Stage 13: Pau to Pau (27.2 time trial)
The only individual time trial of this year's Tour will be a test of strength over a rolling course, but also a test of the head. Go too deep and riders will surely be burned on the mountain tests to come over the weekend. The eventual winner of the Tour has finished in the top four of every long time trial since 2011 but this time they may prefer to save their legs.
July 20, Stage 14: Tarbes to Tourmalet Bareges (117.5km)
The first of the five HC climbs comes on stage 14 - a short, sharp 117.5km race to the summit of the storied Tourmalet. The highest surfaced climb in the Pyrenees has featured in the Tour 87 times but this will be only the second stage finish at the summit.
July 21, Stage 15: Limoux to Foix Prat d'Albis (185.5km)
The second summit finish in a row comes on a more traditional Pyrenean stage, which rolls over several testing climbs, sapping the legs and the minds. However, there is no tradition to the finish with the Prat d'Albis making its Tour debut.
July 23, Stage 16: Nimes to Nimes (177km)
After the second rest day comes a pretty flat stage, though with constant changes of direction on a loop starting and finishing in Nimes, the sprinters' plans could be disrupted if the Mistral wind blows.
July 24, Stage 17: Pont du Gard to Gap (200km)
And so towards the Alps, though stage 17 features none of the big passes on a day of gradual climbing up to Gap. This is the stage in the race where breaks are often allowed to stay clear as the battle for yellow rages behind, but this day has been made to be utterly unpredictable. A punchy final climb of the Col de la Sentinelle could be a springboard, while it's not inconceivable some of the sprinters survive in the main group.
July 25, Stage 18: Embrue to Valloire (208km)
There may only be five HC climbs in the entire Tour, but two of them come in a single day as the riders tackle the Izoard and the Galibier before dropping down to the finish in Valloire. The stage winner should come from whichever group first crests the Galibier - appearing for the 60th time as the Tour's most regular Alpine venue - and this could be a decisive day overall.
July 26, Stage 19: Saint-Jean-de-Maurienne to Tignes (126.5km)
Stage 19 is a leg-sapping day designed to test the resolve of anyone planning a late move in the general classification. The Iseran - at 2,770m the high point of the Tour - comes in the middle of the stage and casts a long shadow before the shorter climb to Tignes offers an opportunity for attacks.
July 27, Stage 20: Albertville to Val Thorens (130km)
The last chance saloon for GC hopefuls does not look an inviting one. The final climb of the Tour is the 33.4km HC slog up to Val Thorens. If ever there was a Tour designed for a true climber, one with only a handful of time trial kilometres and four HC climbs in the last three days could be it.
July 28, Stage 21: Rambouillet to Paris Champs-Elysees (128km)
There are two big questions on the final day. Will the yellow jersey winner choose a few glasses of champagne or, as Geraint Thomas did last year, prefer a beer or two as they celebrate along the roads of the Parisian suburbs? And which sprinters will have survived the mountains in order to battle it out on the Champs-Elysees?