The Lithuanian finished in a time of 5:08.53 to win the 199.2km stage from Royan to Melle in what was a convincing performance.
Kruopis left his rivals trailing in the final sprint, with Adrien Petit of France in second, and Yauheni Hutarovich of Belarus in third.
Marco Haller of Austria was fourth, with Team Europcar rider Yohann Gene of France in fifth.
Thursday’s stage three sees the riders negotiate the significantly shorter 99.4km route from Pleumartin to La Roche Posay.
- Yauheni Hutarovich