Spieth, of the US, had three birdies in a four-hole stretch around the turn, and outside of his lone bogey on the 13th hole, his only disappointment was having to settle for par on his final hole at the par-five ninth.
After undergoing a lengthy lean spell, Spieth feels much more confident in his ability to put four good rounds together.
"Days like today I'm not surprised, but what I will say is I'm confident relative to other time periods I've been off to similar starts, which is a really good place to be," he said.
"I believe I can shoot five- or six-under each day out here. Not to say that that means it'll happen, but there are other times I would be sitting there going, how do I hold this together, to be honest."
Compatriot Harris English finished with 65 in a large group one behind, which also included Olympic silver medallist Rory Sabbatini of Slovakia.
SH Kim of South Korea was five under with two holes to play when the opening round was halted by darkness.
His countryman, KJ Choi, meanwhile, made a rare PGA Tour start at the age of 52. He was in the group at 66 that included Stewart Cink, who turns 50 in May.
Defending champion Hideki Matsuyama, of Japan, opened with a 68, one better than Australian Adam Scott.
Michael Castillo, the Kapalua club pro who qualified for his first PGA Tour event at age 60 while battling cancer, opened with a 79.