Alaphilippe, who won the line race at the World Championships in Italy last weekend, stopped pushing the bike, a beginner's mistake, and Roglic was able to throw the bike forward and win by a few centimeters.
Alaphilippe was then downgraded to fifth place because he did not keep his line on the riot.
"In the end, I managed to win something," Roglic told Reuters.
He looked to be on his way to victory in the Tour de France but lost the lead in almost a minute on the penultimate stage to his Slovenian compatriot Tadej Pogacar, who was also in today's race over 257 kilometers.
- It was unbelievably even.
It shows that you should never give up and push to the last centimeter.
It was the first time I competed in the oldest race in the calendar (run since 1892).
I really wanted to win a "Monument race" and I am so happy that I succeeded, Roglic says to Cyclingnews.
Here is a link to moving pictures from the sprint:
New Italian victory in the Giro d'Italia
Several of the big guns from the Tour de France lined up in the race that went against the second stage of the Giro d'Italia.
In the Giro d'Italia it was a new Italian victory.
Diego Ulissi sprinted down Peter Sagan in a close settlement up to the goal in Agrigento in Sicily.
It was his seventh stage victory in Girot.
Yesterday's tempo winner Filippo Ganna sat safely in the group and kept the pink leader jersey.