Simon Yates claimed his first career Tour de France victory on stage 12 from Toulouse to Bagneres-de-Bigorre.
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The Mitchelton-Scott rider held off Gregor Mulhberger and Pello Bilbao in a sprint after the trio got away from their fellow escapees on the climb of the Hourquette d'Ancizan with a little over 30km remaining of the 209.5km stage.
Simon, the 2018 Vuelta a Espana winner, is riding the Tour to help his twin brother Adam's general classification ambitions, but having lost over an hour in the overall during the first 10 stages he took his opportunity to go after some individual glory.
"I've been saving energy until we got here in the mountains and this was the first chance to try something," he said. "Normally I would be back helping Adam but had my own chance and grabbed it with both hands.
"I wasn't very confident of beating either of them as I didn't know how fast they are. But my director said I had to be in front coming round the last corner so I made sure I did that and thankfully held on to win.
"I'm very proud of winning stages in all three Grand Tours and hopefully there are more to come.
"We'll see if there are more chances this Tour - my main priority is to help Adam. I just had the chance to get up the road today. I'll see how I am in the next few days.
"We're having a fantastic Tour and long may it continue."
Simon did much to drive the final selection on the Hourquette, the second of two category one climbs on the Tour's second mountain stage, as what had been a 40-man breakaway splintered into several pieces.
Bora-Hansgrohe's Muhlberger was able to follow the Mitchelton-Scott man over the summit with Astana's Bilbao not far behind, and the trio raced down the long descent into town together.
The Lancastrian looked perhaps the least likely of the three to win in a sprint given his slight frame, but he used some of his old track racing nous to attack on the way into a corner with 200 metres to go and got the power down on the final straight.
News of Simon's win brought a broad smile to the face of Adam before the peloton came into town almost 10 minutes later.
All eyes had been on Frenchman Julian Alaphilippe of Deceuninck-Quick Step to see if he could hold the wheels of the main contenders over the Peyresourde and the Hourquette and keep the yellow jersey for another day, and he was able to do just that.
However defending champion Geraint Thomas, second in the general classification, will hope to take a chunk out of his 72-second deficit in Friday's 27km time trial in Pau.
Tour de France: Stage-by-stage guide
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?