Koepka, who shared the overnight lead with Norway's Viktor Hovland (25) and Spain's Jon Rahm (28) charged in front with a spotless outward nine highlighted by an eagle on the par-five eighth and a birdie at the second to reach the turn on 10-under.
Resurgent Australian Jason Day (35), who started the day two back of the leaders, was also on the move carding back-to-back birdies at two and three.
Start times were pushed ahead by 30 minutes in an effort to get as much play in as possible before forecasted thunderstorms sweep into Augusta.
Both Koepka, a four-time major champion and Day, the 2015 PGA Championship winner, were among those benefiting from the early starts.
But scoring will be far more difficult on Friday, particularly for late starters like Rahm, who is in the second to last group out with Justin Thomas (29) and Cameron Young (25) at 13:18 pm ET.
While Rahm, winner of three events already this season, was always expected to contend at the year's first major, Koepka was not.
The four-time major champion emerged from the LIV Golf 18-member contingent to plant the rebel circuit's flag atop the leaderboard.
Tiger Woods (47), who has never missed the Masters cut as a professional, will likely need to improve on his opening round two-over 74 if he is to keep his streak alive.
Even in Thursday's superb conditions, Woods slumped to his worst opening round since 2005 and with the rain and cold sure to put his surgically repaired leg to an even bigger test, he will need to find some of the Augusta magic that earned him five Green Jackets.
Woods, who nearly lost his right leg in a 2021 car crash, is in the fourth last group to tee off with Hovland and Xander Schauffele (29).
Rory McIlroy (33), making his ninth attempt at completing the career Grand Slam, opened with a 72 and was out in the morning. He was struggling, however, with bogeys at the second and third leaving the Northern Irishman with even more work to do.
World number one and defending champion Scottie Scheffler took a bogey at the first to drop to three-under, seven back of the pace-setting Koepka.
