Brooks Koepka remained on track for an incredible fourth major win in eight starts when he raced into a commanding seven-shot lead at halfway in the US PGA at Bethpage Black.

The Floridian (29) followed his opening 63 with a five-under 65 to lead by seven shots from Jordan Spieth and Adam Scott on 12-under par — the lowest 36-hole score in Major Championship history— as Graeme McDowell shot a 72 to lead the Irish challenge.

McDowell is tied for 47th on two-over and Shane Lowry fired a 69 to sneak in on the four-over cut mark, tied for 68th, a shot adrift of Rory McIlroy, who is tied 57th.

“This probably sounds bad, but today was a battle,” Koepka said before heading to the driving range to work on his game. “I didn't strike it that good. I was leaking a few to the right. But I think the way I hung in there today and battled it, I think that was probably more impressive than yesterday, not having your A game but still being able to shoot a great score. I was very, very pleased with the way I played today.”

It was another imperious performance from Koepka, who shot out of the gates in impressive fashion, reeling off birdies at the first, second and fourth to get to 10-under to tighten his grip on the Wanamaker Trophy.

He made his first bogey of the week at the tough 10th hole but responded in devastating fashion for the field with birdies at the 13th, 15th and 16th before following a bogey at the 17th with a birdie from 10 feet at the last.

"If he wins this one, that's four out of eight," said Lowry, who had to make made a clutch birdie from eight feet at the 17th to get back to the cut mark, then parred the last for a 69 that left him on four-over par, a shot behind McIlroy who came back from a five-over after three holes to salvage a 71.

"That's like Tiger Woods of early 2000 stuff, Lowry said. "I don't understand how people leave him out of the conversation anymore.  I think right now, he's obviously the best in the world."

The Offaly man started the day tied for 112th after a first round 75, but he slowly clawed his way towards the top 70 and ties.

After rolling home a 25 footer for birdie at the sixth, he bogeyed the 16th but then produced a gutsy finish, hitting a seven iron to eight feet at the 17th to set up a key birdie.

"Yeah, I played lovely," Lowry said.  "To be honest I probably should have shot a better score the way I played.  But look, I did a lot of damage yesterday. Three-putting the ninth hole yesterday was a killer, my final hole, from eight feet, it was disappointing.

"I said to the boys, there's nothing you can do, just go out and try to shoot the best score I can.  I did.  I played nice. When you break 70 around here, it's always a good day."

As Tiger Woods slithered to a disappointing 73 to miss the cut on five-over, Spieth made 141-feet of putts in a four-under 66 to take the early lead on five-under-par.

Scott looked set to overtake the Texan when he holed from outside 20 feet for birdies at his first three holes, then picked up further birdies at the fifth, 11th, 13th and 14th.

The 2013 Masters champion needed to play the last four holes in one-under to shoot an eight-under 62 and match the major championship scoring record set by Branden Grace at Royal Birkdale two years ago.

But he missed from inside three feet for par at the 17th before parring the last for a  64 that left him tied with Spieth on five-under with Daniel Berger, Dustin Johnson, Kelly Kraft, Mat Wallace and Like List sharing fourth place on four-under, eight off the pace.

"I think I'm probably 90 per cent back to where I was at my best," Spieth said of his putting resurgence. "I feel as good or better 15 feet and in. I feel like I’m where I should be."

McDowell was pleased to make the cut and pour more confidence into his game, explaining: "This game is it all about confidence.  We know that.  We see these guys go on streaks where they win tournaments and they just kick on.  

"A guy like Brooks, he's just playing on pure confidence and pure momentum right now, and that's what I'm building right now, you know, the snowball effect.  

"The snowball isn't very big right now but as I continue to play, continue to play solid rounds of golf on the biggest stages, I feel the confidence will continue to grow."

McIlroy produced a scintillating charge on his back nine, recovering from dropping five shots in his first three holes by coming home in 31 for a 71 that left him on three-over.

But it was a week to forget for Pádraig Harrington, who followed a 75 with a 77 to miss the cut comfortably on 12-over par.

The Dubliner was disappointed with his preparation for a major test and plans to pull out all the stops over the next two weeks to be ready for the Canadian Open.

"I'd give myself a Z for preparation," Harrington said. "It was very, very poor and it got shown up. At 47 years of age, you'd think I'd know better. The only good thing is that sometimes you need to be taught a lesson to get motivated to do the right things."

Koepka is doing all the right things.

• Strokes gained off the tee — 3rd

• Strokes gained tee to green — 1st

• Fairways hit — 18th

• Driving distance —23rd

• Strokes gained approach — 1st

• Greens hit — 4th

• Proximity to hole — 2nd

• Strokes gained putting — 12th

• Strokes gained total — 1st

“Yeah, I putted great. I feel every time I slide the putter right behind the ball, it's lined up perfectly, and I don't have to do anything. I don't have to kind of adjust. I feel as comfortable as I've ever felt putting this week. You know, like I said, poa isn't my favourite grass to putt on, but I felt like my speed control has been pretty good.

Today was a little off. I left a few short. You know, and then 17, I actually hit a great putt. Just bounced twice on me. So I'm very pleased with what we've done since Augusta, the changes we've made and where it's at right now.”

Tiger Woods, who finished 17 strokes behind Koepka to miss the cut, was impressed.

Woods said: “What Brooksy did, he's driving it 330 yards in the middle of the fairway. He's got 9-irons when most of us are hitting 5-irons, 4-irons, and he's putting well. That adds up to a pretty substantial lead, and if he keeps doing what he's doing, there's no reason why he can't build on this lead.

“The golf course is soft enough where the power helps. So when he did miss the fairway, I missed it far enough down there where he was still able to hit wedges and 9-irons on the green, and on top of that, he missed on the correct sides. So he had good angles.

” You know, relative to the field, I was about that long, early in my career. You know, when you're able to hit the ball much further than other players, and get on the right golf courses where setups like this is penalising if you are a little bit crooked, and if he does miss it, as I said, he misses on the correct sides, and he's far enough down there to where he was able to get the ball on the green and he did all the little things right.

“He had the good angles. He missed the ball in the correct spots, and it adds up to a big lead and as I said, there's no reason why he can't increase this lead.”