Sweden's Ludvig Aberg, making only his third major start, seized a one-stroke lead in Friday's second round of the US Open as rivals including world No 1 Scottie Scheffler stumbled at formidable Pinehurst.
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World No 6 Aberg, a runner-up in April's Masters in his major debut, stood on five-under par after 10 holes.
He fired a four-under 66 on Thursday.
Aberg, 24, sank a 30-foot birdie putt at the par-4 second hole and an 11-footer for birdie at the par-5 fifth to grab the lead alone at 6-under, but found dirt and weeds left off the eighth tee and made bogey.
A pack on four-under included Belgium's Thomas Detry, who shot 67 to reach the clubhouse on 136 with Bryson DeChambeau, the 2020 US Open winner and runner-up in last month's PGA Championship who fired a 69.
At four-under still on the course were ninth-ranked Patrick Cantlay, an American who shot a 65 to share the 18-hole lead with Rory McIlroy, and France's Matthieu Pavon, like Cantlay seeking his first major crown.
World No 3 McIlroy stood sixth on 137 after struggling to a 74 while Japan's Hideki Matsuyama, the 2021 Masters champion, was another stroke back on 138 after a bogey-free 66.
But pre-Open favourite Scheffler meanwhile was in real danger of missing the cut after carding a 74 to stand on five-over 145.
Meanwhile DeChambeau, among 12 players from Saudi-backed LIV Golf in the field, made five birdies and four bogeys, with a tap-in birdie at 18 keeping him solidly in the hunt.
"Was very happy with how I stayed patient, gave myself good opportunities when they mattered, and I made a lot of clutch putts coming in," DeChambeau said.
"If the wind picks up, it's going to be diabolical."
Detry, ranked 55th, matched his PGA Tour best finish with a runner-up effort in March's Houston Open.
The 31-year-old Belgian had his best major finish last month with a share of fourth at the PGA Championship.
The back-nine Friday starter birdied three consecutive holes before a bogey at 14, then birdied the first, third and fifth holes to grab the lead only to bogey the sixth and eighth to shoot 67 and share third.
"My confidence is good. My play is good. I feel mentally stable as well," Detry said. "I think I've got all my chances. I think it's going to be a fun challenge."
McIlroy, chasing his first major win in 10 years, closed with a bogey at the ninth. He also had bogeys at 11 and the par-3 15th against a lone birdie at the third.
"Had to have your wits about you," McIlroy said. "I wish I had converted a couple more of the chances.
"Still overall in a great position going into the weekend."
SCHEFFLER SOARS TO 74
The four-time major winner from Northern Ireland played alongside top-ranked Scottie Scheffler and second-ranked Xander Schauffele.
At the par-5 fifth, where Scheffler and Schauffele each made double bogeys, McIlroy made a nine-foot par save putt.
Pre-tournament favourite Scheffler was left sweating on whether he would advance to the weekend rounds after his 74. It was the first birdie-less major round of Scheffler's career, the two-time Masters champion undone by bogeys at the par-3 15th and 17th plus his double.
"It was definitely a grind," said Scheffler. "I'm proud of how I fought. I gave myself a good chance. Today I just couldn't get the putts to fall.
"This golf course can be unpredictable at times, and maybe it got the better of me the last couple days."
Schauffele, who won last month's PGA Championship, was four off the lead after shooting 69 to stand on 139 in a share of 10th.
Matsuyama hopes to sustain his momentum.
"I was playing really great," he said through a translator. "My short game was on point. That really helped. Hopefully I can keep that momentum through the weekend."