LE GRAND-BORNAND, France—Tadej Pogacar wagged his head and grinned, his blond hair slick from sweat and rain, his cheeks ruddy from the mountain cold and a colossal effort from smashing his remaining rivals at the Tour de France. As he cooled down on the stationary bike, the defending champion seemed to have even surprised himself. “Ah, what a ride. What a day,” he said, unable to wipe the smile of satisfaction off his face. Pogacar dealt a demoralizing blow on the first day of the Tour in the Alps on Saturday, when cycling’s precocious star claimed the yellow jersey after what was a grueling eighth stage to everyone else. Pogacar started the day 3 minutes, 43 seconds off the lead of Mathieu van der Poel. Five mountain passes and nearly four hours later, Van der Poel finished more than 20 minutes off the pace. The Dutchman relinquished the lead he …