Stage 18, the first in the Alps, was led by breakaway riders throughout the stage's climbs, which included the first-category Col de Vars and the hors catégorie Col d'Izoard and Col du Galibier. The 34-strong breakaway had been reduced to a group of elite riders by the foot of the Galibier, the final climb. Nairo Quintana attacked with still to climb, and by the summit had built a lead of over a minute and a half, which he held on the descent to the finish. Meanwhile, with remaining of the Galibier, Bernal attacked from within the group of general classification contenders containing Alaphilippe and Thomas, allowing Bernal to recover half a minute on his rivals by the finish and move up to second overall. The lead of the mountains classification went to Romain Bardet, who was a pre-race favourite for the yellow jersey but moved out of contention after losing 20 minutes on stage 14's Col du Tourmalet, thereafter switching focus to breakaway rides.