Sagan was followed closely by Belgium’s Van Avermaet of BMC Racing in second and UnitedHealthcare Pro Cycling’s Kiel Reijnen of the US.
“I attacked,” Sagan said. “We were a good group with eight riders. When I was on the front nobody wanted to work with me. After, I said: ‘It’s better waiting for the group so I can do a good sprint’ and I won.”
There were no time bonuses given on Monday so Sagan is tied on time with Van Avermaet and Reijnen.
France’s Tony Gallopin finished fourth and Tejay van Garderen rounded out the top five.
Tour de France winner Chris Froome of Sky Pro Cycling finished 77th, 4 minutes, 59 seconds behind Sagan. Britain’s Froome is racing in Colorado for the first time.
Yesterday’s second stage was a 203km ride from Aspen to Breckenridge, Colorado.