Under rainy, overcast conditions at Augusta National, the second round began with Koepka and DeChambeau level atop the leaderboard after opening on six-under par 66 with Mickelson one stroke adrift.
Koepka, who missed last year's Masters with a left wrist injury, is playing Augusta National for the first time as a major champion, having last year defended his US Open title and won the PGA Championship.
"(It's) just an understanding of how to play and handle the bigger tournaments and understanding how to deal with pressure a little bit better," Koepka said.
"I get to whatever major championship it is and just when I arrive there, I just get a good feeling. I don't know how to explain it. I'm just dialed in and I'm focused the entire week."
Koepka birdied five of the first six back-nine holes on Thursday, including four in a row starting at the tricky par-3 12th on a rain-softened course that figures to be even more vulnerable in round two.
While his best major showing is only a share of 15th at the 2016 US Open, Dechambeau stung Augusta National six birdies in the last seven holes, including four in a row to end his round.
"I was hitting it great all day, driving it well," he said.
"Just an accumulation of great golf that finally showed in the score."
Five-time major champion Mickelson, who at age 48 would become the oldest winner in major golf history if he lifts a fourth green jacket this week, birdied five of the last seven holes Thursday.
"The greens are softer than they've ever been and they're not as fast as they normally are," said the US left-hander.
DeChambeau tees off at 10:42 a.m. (1442 GMT) Friday with Mickelson in the group behind him and Koepka in the one after that.
World number two Dustin Johnson and England's Ian Poulter, who also have early starts, shared fourth on 68 with 2013 winner Adam Scott of Australia, Spain's Jon Rahm, South Africa's Justin Harding, Thailand's Kiradech Aphibarnrat and American Kevin Kisner in a pack on 69.
The past 13 Masters winners all were in the top-10 after round one, a good omen for those within three of the lead.
- Tiger, Rory start late -
But Woods, a 14-time major winner seeking his first major triumph since the 2008 US Open, was hopeful of staying in the hunt after an opening 70 -- the same score he opened with in three of his four Masters victories.
Since returning last year from spinal fusion surgery, Woods has contended for major titles at last year's PGA Championship and British Open and ended a five-year US PGA Tour win drought at the Tour Championship.
"The whole idea is to try and peak for four times a year," he said.
"And so I feel like my body's good and my game's good, it's sharp, so just got to go out there and execute."
Woods tees off in Friday's penultimate group with four-time major winner Rory McIlroy in the final group. McIlroy opened on 73 in his quest to win his first green jacket and complete a career Grand Slam.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)