On the first of three brutal days in the Dolomites, French veteran Pinot, who is retiring at the end of the season, drove the breakaway on as he collected enough points to take the King of the Mountains classification lead.
However, he lost out on a stage victory in the final climb to the line for the second time in a week.
Zana, the reigning Italian champion, is part of the Jayco–Alula team and was racing on home roads in his home region in north-east Italy.
Down the mountain, Roglic, paced by team-mate Sepp Kuss, went on the attack from the group of leading riders
The Jumbo-Visma pair quickly dropped all their main rivals except Welshman Thomas of the Ineos team who stuck to Slovenian Roglic's rear wheel and celebrated his birthday by keeping the leader's pink jersey.
Thomas, the 2018 Tour de France champion who is bidding to become the oldest-ever Giro champion, remained 29 seconds ahead of Roglic who took second place from Joao Almeida.
Friday's 19th stage is a 183km ride from Longarone to Tre Cime di Lavaredo that includes five significant peaks.
Saturday's penultimate stage is a ferocious 18.6km uphill time trial.