The Dimension Data team worked to set up Mark Cavendish for the sprint finish at the Tour of California but a late shuffle in the peloton left the Manxman stuck in traffic and too far back to contest the sprint.
The wide roads in the final kilometre of the finish in San Diego allowed for a hard-fought sprint as riders constantly moved up from behind only to suffer into a headwind. Sagan timed his move well, following the wheels through to the front before making a late surge to the line.
Cavendish, who won four stages and the points classification at last year's race, was led out by two teammates but, like many, had to hit the brakes in sight of the finish line.
"We wanted to win today’s stage with Mark Cavendish. This was our goal and we worked hard for it, which was good and nice to see," directeur sportif Rolf Aldag explained.
"The guys stayed at the front of the bunch for most of the race and helped control the gap to the breakaway. On the run-in to the finish we were positioned well. Unfortunately, Mark got boxed in and couldn’t open his sprint. But that’s racing, I guess.
"It was only the first stage today, so we will continue to show ourselves. There are still a few chances to get a stage win."