BMC rider De Marchi had all the answers in the miserable conditions as he left the pack trailing before a classy finish in the final uphill stage gave him victory in the 101st staging of the autumn classic.
De Marchi attacked with around 20km remaining of the race with Rigoberto Uran never seriously threatening his domination on the last climb to San Luca as he finished a distant second, around seven seconds behind.
Dylan Teuns finished third for BMC.
“I did not think I could do it, I risked and it went well,” said De Marchi.
“I thought about it for a long time, I wanted to try a business from afar, even if this race is not very suitable for my characteristics and this year the starting lot was of the highest level.
“I was guided by instinct. On the last lap I knew I could play it, I did not have to do stupid downhill to get to the feet of San Luca with a margin that could allow me to resist the group’s comeback. I did it.”