Brooks Koepka may have set a new course record at Bethpage Black but the defending champion felt he could have gone even lower in his opening round at the US PGA Championship.

Having started out on the back nine, Koepka signed for a blemish-free score of 63 as he overshadowed playing partners Tiger Woods and Francesco Molinari on Thursday.

The American carded seven birdies to set the early pace in the clubhouse, though he insisted afterwards that a 60 was possible, had he taken his opportunities.

He had three missed birdie putts from 10 feet or less, as well as failing to capitalise on the two par-five holes.

"Well, I didn't take care of the par fives, didn't birdie any of the par fives," Koepka lamented to the media. "That was disappointing, because I felt like, you know, those are holes you should be able to birdie.

"Definitely can reach, what is it, four and [I] just hit a bad drive there. And then 13 I can get there too, I just hit it in the bunker.

"And then the second hole today, my 11th hole, I missed about a five-footer. That would have been nice to shoot 60. I guess that would have been pretty good."

Koepka, who turned 29 towards the start of the month, is chasing a fourth major, having defended his U.S. Open title last year following his maiden victory at Erin Hills in 2017.

"I think I'm still learning, understanding my game, and I've figured it out, and I think, over the next few years, I'm excited for what's to come," he added.

"I understand a lot more about my misses, where to hit it, and major championships I just suck it up, and you don't always have to aim at the flag like you do in regular events. Sometimes it's just about how few bogeys and doubles you make this week."

Playing alongside Woods - who finished at two over - made sure plenty of spectators followed Koepka's stunning exploits, though he welcomed the presence of a vocal following in New York.

"I think it's fun when they're getting loud, they're rowdy," he said of the fans. "I enjoy when they can really get behind you. You definitely want them on your side this week, that's for sure."