Chase, the younger brother of 2017 US Open champion Brooks, produced a brilliant display at Glendower Golf Club, making seven birdies, one eagle and two bogeys for an opening seven-under-par 65.
The 24-year-old has been joined on -7 by home favourite Branden Grace, who made an impressive three eagles, three birdies and two bogeys, and the leading pair are one stroke ahead of England's Chris Paisley in third.
Koepka reserved particular praise for his iron-game during Thursday's round and he believes this played a huge part in him being able to set up so many chances of making birdie.
"It was great," the European Tour rookie said. "I think I played flawless, except for the second shot on the first hole. I hit a poor shot there and saved it with a 12-footer for a bogey. I made eagle on the second hole and kind of kept it going from there.
"Inside 150 yards, I was really good today. I didn't have too many shot from inside 150, but every one that I had, I was hitting them close. The putter was really hot on the front nine and then I had a bunch of short putts on the back nine which helped a little bit."
South African duo Erik Van Rooyen and Richard Sterne are sitting two strokes off the pace on -5 and they are joined on that mark by Scotland's Neil Bradley and Spain's Nacho Elvira.