The Italian completed the 156km route that started in Cuneo in a time of 3:41:49 and was followed home by Damiano Caruso (Bahrain-Merida) and Eddie Dunbar (Team Ineos).
“I’m a bit disappointed really, I wasn’t the quickest but I was certainly one of the strongest,” said Dunbar.
“At the finish I had Gianluca [Brambilla] and Eros [Capecchi] there and I knew them being Italian they’d be a bit more keen to get to the finish than me so I gambled a bit and then got caught by the two guys behind but that’s bike racing I guess. You live and learn.
“At the end it was literally who can push the biggest gear, the other guys had the experience on me and that extra bit of endurance but it’s a start for sure for me.”
With 1.7km remaining Brambilla led and was soon joined by Dunbar and Capecchi.
Cappechi then took over the lead, followed by Brambilla and with 800m to go the former led it out.
Caruso and Benedetti joined and there were five leaders with 300m to go.
Brambilla launched an attack, but he was gone too soon. Benedetti powered to the line, taking his first professional victory.
Earlier, with 32km to go, Dunbar, Capecchi, Brambilla and Caruso went clear from a chasing group and crested the top of the Montoso climb, which was 8.8km in length with an average gradient of 9.5 per cent.
Benedetti soon joined the leaders, which made it five up front.
Dario Cataldo (Astana Pro Team) and Polanc were on their heels.
The duo, as well as Matteo Montaguti (Androni-Giocattoli), joined the leaders with 20km remaining.
The octette rode well together until Cataldo was dropped.
As the leaders hit the final little climb before the finish, Benedetti led before Brambilla hit the front close to the flamme rouge.
Polanc, who was dropped from the break in the final kilometres, crossed the line shortly after Benedetti to ride into the maglia rosa with a considerable lead on the general classification.
The 27-year-old Slovenian, who leads Roglic by four minutes and seven seconds, placed sixth in this year’s Tour of Turkey, Tour of Oman and Giro di Sicilia.
“It’s always a dream to wear the pink jersey. For me after two stage wins at the Giro I think this was a dream for us,” Polanc said.
“For the stage tomorrow it will be big for the team. We didn’t come here with GC ambitions but we’ve had Valerio [Conti] in pink for quite a few days and now for me it’s also really nice.”