Geraint Thomas said he had a "decent day" on his 37th birthday as he kept the Giro d'Italia's leader's pink jersey and withstood an attack by Primoz Roglic that shook off all the other leading contenders.

On the first of three brutal days in the Dolomites, local boy Filippo Zana edged French veteran Thibaut Pinot in an exhausted dash to the line to win the 18th stage of the race.

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 Ineos' Welshman Thomas 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, is bidding to become the oldest Giro champion.

Italian Fiorenzo Magni was 34 years and 180 days old when he won in 1955.

UAE Team Emirates' Joao Almeida dropped down to third overall, after finishing ninth near the Austrian border.

"Decent day," Thomas said.

"To gain time on Almeida and not get dropped by Primoz is a good day — a solid day for sure."

"I felt pretty good, pretty in control," he said of his battle with Roglic.

"In the last two kilometres he really squeezed on again. He was really strong but I'm happy with how I was," Thomas added.

He remained 29 seconds ahead of Roglic who took back the second place he lost to Joao Almeida on Tuesday.

"Primoz had a bad day the other day, Almeida today. I've just got to keep being consistent. Just take it day by day, climb by climb," Thomas said.

After the top three there is a gap to Irishman Eddy Dunbar who is fourth at two minutes and 39 seconds.