The fast men battled it out on a tough uphill finish in Morro Bay, with Boswell finishing in the pack to remain in a podium position, 14 seconds back on the yellow jersey.
A testing run-in and lack of organisation in the bunch made for a tricky sprint, with Team Sky unable to put together a run to the line. Elia Viviani led the team home but found himself too far back to launch a sprint.
The team had stuck together and came to the front en masse with 15km to go, as the day’s five-man break were pulled back by the bunch. A group finish also allowed Tao Geoghegan Hart and David Lopez to keep their GC footholds ahead of more selective stages to come.
Bora-Hansgrohe had patrolled the front for much of the day and their work was rewarded by a victory from world champion Peter Sagan. The Slovakian was a class above on the ramp to the line, holding off Rick Zabel (Katusha) and Simone Consonni (UAE-Fly Emirates).
Rafal Majka (Bora-Hansgrohe) continues to lead the race by a slender margin of two seconds over George Bennett (LottoNL-Jumbo).