Shane Lowry broke the course record and raced to the top of the leaderboard on home soil at the Amgen Irish Open, as Rory McIlroy was left feeling "frustration" in his putting.
Lowry mixed nine birdies with a lone bogey during a brilliant eight-under 62 at Trump International Golf Links & Hotel, lifting him to 11 under and three strokes clear of the rest of the morning starters.
The former Open champion's closest challenge from his half of the field came from Haotong Li and Jacob Skov Olesen, who posted rounds of 63 and 64 respectively to move to eight under, with Tom McKibbin among those six strokes back.
Olesen's fellow Dane Niklas Norgaard Moller joined his compatriot and the Chinese star at eight under, to make it a three way tie for second.
Patrick Reed heads into the weekend on four under, with Jon Rahm and Robert MacIntyre another shot adrift, with Rory McIlroy nine off the halfway lead after struggling on the greens during a second successive 69.
First-round leader Joaquin Niemann followed his opening-round 65 with a level-par 70, while Tyrrell Hatton was among the notable names to miss the cut.
Lowry, who won this tournament in 2009 as a 22-year-old amateur, was playing in the group behind McIlroy and saw crowds for him grow after a run of three consecutive birdies from the third.
He holed a long-range putt at the seventh and picked up further shots over the next two holes to race to the turn in 30, then added back-to-back birdies from the 11th to leave Lowry briefly on '59 watch'.
Lowry's hopes of a sub-60 round suffered a setback with a three-putt bogey from 20 feet at the 13th, although he bounced back by birdieing the 15th and closed out his record-breaking round with three straight pars.
"It was real fun out there," Lowry said. "I got off to a nice start and then I just kept going. I felt great out there. Just kept my foot on the gas and tried to keep hitting good shots and keep holing putts."
McIlroy holed a birdie from off the second green but failed to take advantage of multiple chances on his front nine, with the world No 2 failing to get up and down from the sand and missing a close-range putt to save par at the seventh.
The Masters champion responded with a sensational eagle at the par-five eighth, firing his approach close, only to card a sloppy bogey at the 11th and cancel out a long-range birdie at the 17th with a disappointing three-putt at the last.
"Frustration," was McIlroy's assessment of his second round. "I just didn't have it on the greens. I really struggled to read them. It's hard whenever you struggle to read them.
"It's been a tough one on the greens the last two days, but tee to green I've actually been pretty happy with how everything else is. I just need to try to hole a few more over the weekend."
MacIntyre carded a 65 to stand at three under and five-time major champion Koepka - who followed his opening round of 75 with 66 - avoided missing the cut despite his Thursday struggles.
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