I'm living a dream I think," said Merlier, who has now claimed maiden stage wins at both the Tour and Giro d'Italia this year.
Pello bilbao wins stage 4, simon yates builds lead, chris froome in breakaway
Cycling Tour Of Volta ao Algarve Postponed as Portugal battles COVID-19 surge.
Laura Kenny also lands silver in the European Track Cycling Championships in Plovdiv.
The 32-year-old has agreed a two-year contract extension with Israel Start-Up Nation.
Ecuadorian Richard Carapaz was second overall with Great Britain’s Hugh Carthy finishing third.
Initial plans had been to retire.
The final kilometre of the 157km stage from Castrillo del Val to Aguilar de Campoo saw the Deceuninck-Quickstep rider surge to apparent victory.
Britain has four Grand Tour debutants in the Vuelta.
Geoghegan Hart scored a surprise win in the Giro d’Italia after starting out aiming to support Geraint Thomas.