SAN DIEGO, Calif., June 17 (Reuters) -- Russell Henley matched his U.S. Open career low to grab the clubhouse lead at Torrey Pines while Phil Mickelson's bid to complete the career Grand Slam in his home city was in peril after a fog-delayed start to the first round on Thursday.

World No. 63 Henley, who entered the week riding a string of poor results, birdied his final hole for a four-under-par 67 that left him one shot clear of former British Open champion Francesco Molinari.

Twice champion Brooks Koepka was a further shot adrift with Xander Schauffele, who as the hometown boy was enjoying plenty of support from the limited spectators in attendance.

"I just kept playing solid, hitting fairways, giving myself birdie looks, and felt good with the putter," said Henley. "Felt like I was reading the greens well and hitting my lines. Just hung in there."

The start was delayed by 90 minutes as players were greeted by a blanket of fog that covered the South Course, which sits atop the cliffs overlooking the Pacific Ocean, and ensured that first round would not be completed on Thursday.

Newsletter signup for email alerts

Henley, who went out in the fourth group off the first tee, started his day with a bogey but was nearly flawless the rest of the way as he worked his way around a challenging course with narrow fairways lined by punishing rough.

Seeking his fourth PGA Tour win and first in over four years, Henley capped his round in style when he reached the par-five 18th green in two for a routine two-putt birdie.

Four-times major winner Koepka, who finished runner-up at last month's PGA Championship, started on the back nine and was cruising along at four under and alone in front until a bogey at the par-three third where his tee shot landed in a penalty area.

World No. 10 Koepka then added another bogey at the par-four seventh following a wayward tee shot and could not close the gap over his final two holes.

"Yeah, I mean, you can't win it today but you can definitely lose it," said Koepka. "Pretty pleased. Not the best, but I'll definitely take it."

Mickelson struggles

PGA Championship winner Mickelson, playing a day after his 51st birthday and a month after becoming golf's oldest major winner, made two late bogeys en route to a four-over-par 75 that left the record six-times runner-up eight shots back of Henley.

Mickelson, who grew up playing city-owned Torrey Pines, started his day at the par-four 10th where he failed to get up and down from a greenside bunker and made bogey.

The left-hander took a penalty stroke at the par-five 13th after his approach shot settled in a bush near the green. His next shot hit the flag stick and ricocheted back into the rough from where he got up and down for bogey.

Mickelson dropped another stroke at the par-four 15th where he struggled with the thick rough lining the narrow fairways but got a stroke back at the par-four 17th where he drained a nine-foot birdie putt but was undone by two late bogeys.

"I had some chances to get the round a little bit better. Fought hard, made a lot of short putts to kind of keep myself in it and then I ended up bogeying six and seven," said Mickelson.

"Two-over would have been a pretty good round and I ended up at four, so I'm a little disappointed about that."

The thick fog eventually gave way to sunshine and the greens firmed up in stronger winds which made for more difficult scoring conditions as the day went on.

Defending champion Bryson DeChambeau went out late in a group with Masters champion Hideki Matsuyama and 2020 U.S. Amateur champion Tyler Strafaci.

Also among the late starters is a high-profile group featuring world No. 1 Dustin Johnson, Northern Irishman Rory McIlroy and Englishman Justin Rose