The Slovak Cannondale rider was simply too fast for Greg Van Avermaet of BMC Racing as he charged to the line at the end of the 98km stage from Aspen to Snowmass
The USA's Kiel Reijnen (UnitedHealthcare) came third.
"Maybe I was a favorite for people, but for me I am very surprised," said Sagan.
"I am very happy. Thank you to all of my teammates, because they were on the front and pulled the group for the whole of the race. And we are very happy when we took the victory for the team.
"I felt very good,. I think this helped me, and maybe it was key for the win today. It was hard, but the climbs were not very long.
"We did tempo and on the last loop we rode hard on the last climb. The last 10km was good for me."
The seven stage race concludes on Sunday in Denver.