South Africa's Haydn Porteous and American Patrick Reed were a stroke further behind on six-under, with world number one Dustin Johnson four strokes adrift after shooting 68 in sublime conditions for golf.
"I think I played really well, putted very well," Koepka said after his round.
"I feel like I started really well, drove it really well. Any time you're making putts ... you're going to shoot good."
The American enjoyed a stellar 2016-17 on the U.S. Tour, claiming his maiden major title in June and capping the season with a role in the United States' Presidents Cup victory.
Starting his round at the 10th hole, the 27-year-old rolled in a 21-foot putt for eagle on the 18th before stumbling with a bogey on the par-four third.
He responded brilliantly to the setback, however, landing his next tee shot two feet from the pin for a tap-in birdie.
After another birdie at the par-four seventh gave him a share of the lead, he rolled in a delightful downhill putt from the fringe of the green at the par-five eighth to move clear and held steady with a par on the last.
Long-hitting Johnson, chasing his third WGC title of 2017, had a slower start in his season-opener. He notched six birdies but was left cursing his second bogey of the day at the eighth, his penultimate hole.
Johnson finished in a big group on four-under that included Swede Henrik Stenson and Spain's Rafa Cabrera Bello.
Former U.S. Open winner Justin Rose was three off the pace on five-under, along with American Matt Kuchar after the pair opened with 67s.
Former world number one Jason Day shot a 69 to be five behind after enduring a mostly frustrating day on the greens.
Hideki Matsuyama suffered a horror triple bogey seven at the third in a scratchy round of 74 and has plenty of work to do over the next few days if he is to successfully defend his title.
(Reporting by Ian Ransom in Melbourne; Editing by Peter Rutherford)
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