McIlroy, who arrived at The Country Club outside Boston fresh off a successful title defence in Canada, used a red-hot putter to card a first-round three-under-par 67 that left him level with Englishman Callum Tarren and Swede David Lingmerth.
Four-times major champion McIlroy, who started on the back nine, was one shot off the lead when disaster nearly struck as he caught a bad break at the par-four fifth hole where his tee shot settled in the tall grass on the lip of a bunker.
The Northern Irishman advanced his second shot 14 feet into another bunker, prompting him to slam his club twice into the sand in frustration before an impressive up and down that he capped with a 13-foot par putt.
McIlroy birdied the seventh and eighth holes to sit alone atop the leaderboard before failing to sink a 13-footer for par at the ninth.
World number 445 Tarren, competing in his second major, eagled the par-five eighth, his penultimate hole, after hitting his approach shot to within five feet.
“I’m kind of pinching myself,” Tarren said. “I didn’t realize it was on the top of the leaderboard until I pulled that final putt on the ninth hole.”
Defending champion Jon Rahm was three shots off the early pace and level with a group including 2021 British Open winner Collin Morikawa and Australia’s Adam Scott.
Jordan Spieth, who had limited practice the last two days because of a stomach bug, bogeyed three of his first four holes but managed to salvaged a two-over par 72.
MICKELSON WATCH
Six-times major champion Mickelson, playing on his 52nd birthday, was scheduled to tee off at 1:47 p.m. ET (1747 GMT) in the company of former British Open winners Louis Oosthuizen and Shane Lowry.
Mickelson arrived at the U.S. Open facing an onslaught of questions regarding his involvement with the lucrative Saudi-backed LIV series, which reportedly paid him $200 million to join.
The long-time fan favourite, U.S. Open runner-up a record six times, needs a win this week to complete the career Grand Slam of golf’s four major titles.
Masters champion Scottie Scheffler, the world’s top-ranked player, and PGA Championship winner Justin Thomas were also among those in the afternoon wave.
Among the 156-player field at Brookline are 13 golfers who played in the LIV event last week and another two — Bryson DeChambeau and Patrick Reed — who will make their debuts on the breakaway circuit in two weeks.
Three-times U.S. Open champion Tiger Woods, who returned to competition this year from his February, 2021 car crash and played the year’s first two majors, is absent this week in a bid to rest his body ahead of the July 14-17 British Open.
