Brooks Koepka lines up to putt on the first green during the first round of the Byron Nelson golf tournament. Photo: Ryan Michalesko/The Dallas Morning News via AP

LOS ANGELES Brooks Koepka warmed up for his defence of next week's PGA Championship with an opening round six-under-par 65 at the AT&T Byron Nelson in Dallas, Texas on Thursday.

Koepka, who will be looking to add a fourth major to his resume at Bethpage Black in New York next week, carded eight birdies and two bogeys to finish the day two off the early pace.

The 29-year-old's round might have been better had it not been for two bogeys on the back nine which were the only blemishes on an otherwise impressive day's work at Trinity Forest Golf Club.

"The last few weeks I've been even impressed with myself and how well I'm striking it," Koepka said. "Controlling it nice and then from there just try to feel like you see some putts go in the hole.

"I think it's important to kind of feel like the hole is opening up with a major. Obviously the goal is to win (here) but if you don't, you want to leave feeling good for next week."

Koepka, who teed off on the 10th hole, made a 41-foot birdie putt on the 18th before draining a 19-foot effort at the par-three second hole.

With wet conditions making for low scoring on a soft course, Koepka was one of nine players tied for fourth on six under, a group which included Sweden's Jonas Blixt, South Korea's Kang Sung-ho and Australia's Matt Jones.

Denny McCarthy of the United States posted the day's lowest score, taking a one-shot lead with an eight-under-par 63.

McCarthy started slowly, and drifted out to two over for the day after a double-bogey six on the par-four fourth.

But the 26-year-old from Maryland surged up the leaderboard with five straight birdies from the sixth hole onwards.

Further birdies followed on the 12th, 13th and 14th before back-to-back birdies on 16 and 17 took him to eight under.

McCarthy attributed his remarkable run of 10 birdies over the final 13 holes to a relaxed approach after a nervous start.

"I came out with a mindset that I wanted to be aggressive, and I wasn't," McCarthy said. "After that I just told myself that I was going to play carefree today, have fun with it and let it go, and I did it.

"I’m pretty proud of myself for just being able to let my mind go after a double-bogey like that."

Tyler Duncan and Tom Hoge were tied for second after posting seven-under-par 64s.

AFP