Spieth and Kirk - who also had seven birdies - set an early first-round target at Waialae Country Club in Honolulu, Hawaii, where Montgomery seized his share of the lead with a birdie at the 18th shortly before play was halted.
Spieth said he was confident he could maintain the pace, something that hasn't always been the case in recent years as his game hit the doldrums and he fought to make changes.
"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," said Spieth, who ended a nearly four-year title drought in 2021 and added a title at the Heritage last year.
"I believe I can shoot five- or six-under each day out here," Spieth said. "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."
Spieth opened with a birdie at the 10th and added another at 12. He rebounded from a bogey at 13 with birdies at the 18th and first, then rolled in an eight-footer at the fourth and put himself inside two feet for a birdie at the fifth.
He picked up one more shot with a 27-foot putt at the par-three seventh.
"I feel like I left a couple out, but I also really had fun today," Spieth said. "It can be a real fun golf course once you start peeling it down these fairways."
Kirk, back in action after a break of almost two months, said he'd been working on his fitness during the downtime.
Kirk, who finished tied for second at Waialae two years ago as he mounted a comeback to the tour, is seeking a fifth US PGA Tour title and his first since 2015.
"I've had a lot of good finishes here," Kirk said. "It's just a place where I'm comfortable."
Montgomery, a Korn Ferry Tour graduate chasing his first PGA Tour title, had joined the leaders with a birdie at the 14th, then bogeyed the 15th before his two-putt birdie at the par-five 18th.
Six players were in the clubhouse a stroke behind the leaders: Harris English (33), Denny McCarthy (29), Brendon Todd (37), David Lipsky (34), Rory Sabbatini (46) and Ben Griffin (26).
South Korean Kim Seong-Hyeon (24) was also five-under with two holes to play when darkness fell.
A big group on 66 included South Korean veteran KJ Choi (52) as well as American Stewart Cink (49).
Defending champion Hideki Matsuyama (3of Japan, who said before the tournament he was still battling the neck trouble that nagged him for much of last season, carded a two-under-par 68.
