The Netherlands' Tom Dumoulin remains third at 1:50, while Slovenia's Primož Roglič is fourth at 2:38. Many days ago they already said that this was the stage for me and that I should go out in the break with Valgren.
Thomas, a former Olympic champion in track cycling, was dramatically booed off the podium on Thursday after claiming his second successive stage win in the high Alps to reinforce his overall lead.
At the finish line in Carcassonne, the Astana rider was the strongest, taking his first Tour stage win.
France's Julian Alaphilippe, holder of the polka dot jersey, was the fastest in the climb but he made his move slightly too late and finished six seconds behind, just ahead of Stuyven. "I wouldn't know what it was".
It was the first career victory at the Tour for Fraile, who rides for the Astana team.
It is Astana's second consecutive win on the race after Spaniard Omar Fraile capped an audacious attack on the final climb of stage 14 to triumph in Mende. The Basque rider also won the mountains classification in the Spanish Vuelta in 2015 and 2016.
The hilly 188-kilometre (117-mile) route from Saint-Paul-Trois-Chateaux in southern France passed through the Ardeche gorges, home to cave paintings dating back some 36,000 years.
Sagan, of the Bora team, is one of the few real sprint specialists still in the race after surviving an Alpine stage trilogy that proved fatal to the hopes of rivals Mark Cavendish, Marcel Kittel, Andre Greipel and Dylan Groenewegen.More news: Jupiter has 12 new moons, 79 total in orbit: Scientists
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"I 100 percent believe in myself and the team, that we do everything in the right way, along with the majority of the peloton as well", said Thomas. "If we go against each other and Dumoulin wins then we would look really stupid".
Sagan, however, went through the first three mountain stages with relative ease and had enough gas in the tank for the final effort in Valence, where he outsprinted Kristoff and Demare into second and third place respectively.
A sea of fans lined both sides of the road up the Cote de la Croix Neuve, and the crowd was disappointed when French contender Romain Bardet couldn't keep up with the leaders.
Britain's Geraint Thomas, wearing the overall leader's yellow jersey, climbs with the pack during the fifteenth stage of the Tour de France cycling race over 181.5 kilometers (112.8 miles) with start in Millau and finish in Carcassonne, France, France, Sunday July 22, 2018.
Eventually, the peloton came back together, although by then a large group of 32 breakaway riders, including Fraile, formed ahead.
The three came to the final kilometre together, but Nielsen proved to be too strong for his rivals and won the sprint easily to make it two wins in a row for the Kazakhstani squad.
"In the last few kilometres", Fraile said.
Associated Press writers Ciaran Fahey and Joseph Wilson contributed.