Pogacar took second place by beating chief rival Jonas Vingegaard in a two-man sprint finish, one minute and 12 seconds behind Ineos Grenadiers rider Arensman, extending his lead over Dane Vingegaard in the general classification by six seconds to 4:13 at the end of the Pyrenean stage.
Vingegaard attacked several times in an attempt to drop Pogacar, but the world champion did not flinch and easily beat his rival in the final metres to further cement his domination.
The day belonged to Arensman, however, as the Dutchman went solo from the day's breakaway in the penultimate climb to the Col de Peyresourde (7.1 km at 7.8%) before his team car hit and knocked down a spectator amid the usual roadside chaos on the Tour.
Arensman never looked back and held firm on his way up to Superbagneres (12.4 km at 7.3%).