Brooks Koepka flexed his muscles and opened with a course record 63 to leave Rory McIlroy spluttering his exhaust fumes in his US PGA defence at Bethpage Black.

Seeking his fourth major win in his last eight starts, the unflappable Floridian brought the beast to its knees with a hugely impressive seven-under effort leaving him nine shots clear of McIlroy and his playing partners Tiger Woods and Francesco Molinari.

He opened his assault by dropping a 40 footer at the 10th and then closed it out by slotting home a 33-footer at the ninth.

In contrast, McIlroy had to wait until the 18th to make a birdie, racking up 35 putts in a two-over 72 as Pádraig Harrington struggled to a 75, hitting only five fairways.

“I said to Harry going up to the 18th tee, ‘I can't remember the last time I played a round of golf without a birdie’. I was like, I better birdie this last hole. Thankfully I did. It was nice to finish that way,” said McIlroy, who was second for strokes gained off the tee but 115th for strokes gained putting.

Iit was either wrong speed at some times or sometimes just a little bit of line, either way, you know, high or low. And just keep it hitting good putts. That's all you can do.”

Masters champion Woods and Open champion Molinari carded a pair of two-over 72s alongside Koepka, who leads by one-stroke after New Zealander Danny Lee birdied the last two holes for a six-under 64.

But it was a struggle for most of the field with England's Tommy Fleetwood ( 67) and Luke List, Chez Reavie, Sung Kang, Pat Perez and the Frenchman Mike Lorenzo Vera (68) among the few to get the better of the Farmingdale monster in benign, sunny conditions, 

McIlroy hit just three fairways on the front nine and turned in two-over as he bogeyed the first and three-putted the eighth.

His putter was also ice cold and after missing chances from 10 and 12 feet at the 12th and 13th, he airmailed the 15th and made bogey, then missed a 13 footer for birdie at the 16th before finally making a five footer for a closing birdie.

“I guess it's early in the tournament,” McIlroy said. “Keep hitting good shots. Don't let it frustrate you. Keep hitting good putts. Eventually things will turn. Hopefully that birdie on the last was the turning point, finish on a positive note and come back tomorrow and hopefully get into red figures for the tournament.”

Koepka's seven-under bogey-free round — his second 63 in the US PGA since last year's second round — was so impressive that Graeme McDowell believes we're seeing the anointment of the next world number one.

"I think he is the best player in the world at the minute," said McDowell, whose boyhood pal Ricky Elliott caddies for the American. "Aside from Rory, he is one of the most talented players I have ever seen. Rory is just pure natural talent, but this guy is pure brute talent.

"Going out with Tiger this morning, a lesser man would have wilted, but the guy has something a little special and I am really impressed by him."

Starting on the back Koepka (29) followed his opening birdie at the 10th by making a 20 footer for a two at the 14th and a six-footer for another birdie at the 18th.

In contrast, Woods turn in three-over 38 as he sandwiched a birdie at the 15th between double bogeys at the 10th, where he overshot the green with his third, and the 199-yard 17th, where he was bunkered and then three-putted.

The 15-time major winner recovered spectacularly by starting his back nine birdie-birdie-par-eagle to go from 58th to fifth but then bogeyed three of his last five holes, three-putting twice.

"I fought my way back around there, and unfortunately I just didn't keep it together at the end," Woods said.

Considering Koepka missed birdie putts inside 12 feet at the 11th, 13th and second, a 63 was almost a modest return.

"He played well," said Woods, who had 31 putts. "I mean, he hit a couple of loose tee shots today that ended up in good spots, but I think that was probably the highest score he could have shot today."

Koepka didn't birdie either of the two par-fives as he hit 14 fairways and 14 greens in regulation and used the putter just 25 times, making 121-feet of putts.

"The only thing you have to do is hit fairways," said Koepka, who only has eyes for extending his lead. "If you're going to hit fairways, you're going to be able to hit the greens and get the ball close to the hole. 

"I mean, obviously my length is an advantage here. But I mean, the one thing I didn't do is take care of the par-5s."

What's even more ominous for Koepka's rivals is that he has no intention of backing off.

"You know, you'd love to make it more difficult for them," he said. "It's always nice being out ahead. I don't think it's happened to me that often where I've got a, what is it, three, four-shot lead over the field as of right now. 

"But you take a hole off, it could change very quickly out here. So you've just got to keep the pedal down."

McDowell believes that aside from his obvious gifts, what sets Koepka apart is his ability to motivate himself for the biggest events.

"It is an amazing mindset he has and he seems to do it from the point of view of creating a chip on his shoulder, even if there isn't one, he likes to try and find one," McDowell said. 

"He likes to try and pull a quote from Brandel Chamblee or whoever it may be. He finds a way to focus his emotions to play well on the big weeks, which is an incredible talent to have, outside the obvious ball striking capabilities. He is pretty good that boy."

McIlroy insisted he wasn’t intimidated by seeing Koepka post seven-under as he warmed up for his opening round.

“All I do is concentrate on myself,” he said. “I've been out here long enough to know that a first round score is just a first round score. The golf course is hard enough without looking at other people. I'm just trying to do my own thing.”

He struck the ball well enough from tee to green to shoot a sub-70 score and that’s given him hope for today.

“Yeah, 63 -- and there's a 64 out there as well from Danny Lee,” he said. “So there's a couple of low scores. I mean, look, if you can put the ball in play and give yourself chances, I felt like I gave myself enough chances today to shoot something sort of in the mid-60s. But, yeah, I mean, it gives me hope. It gives me hope I can go out tomorrow and shoot a low one.”