Louis Oosthuizen joined Russell Henley in a share of the lead as the opening round of the U.S. Open was interrupted, while Phil Mickelson's quest for back-to-back majors got off to an awful start at Torrey Pines.

A fog-enforced delay meant the start of the major tournament was pushed back by around an hour and a half on Thursday, and while Oosthuizen was unable to finish his round, the 2010 Open Championship winner still ended the day alongside Henley atop the leaderboard.

Oosthuizen – one of three players to finish in the top 10 at each of the last two U.S Opens, joining Rory McIlroy and Xander Schauffele – moved into a tie at four under after birdieing the 14th hole in San Diego.

Henley had set the early pace after claiming an early lead behind his impressive four-under-par 67, which was enough for him to initially head back to the clubhouse with a one-shot lead over Francesco Molinari and Rafa Cabrera Bello.

Round 1 of the #USOpen was suspended due to darkness.

T1. @RussHenleyGolf -4 (F)
T1. @Louis57TM -4 (16)
T3. @F_Molinari -3 (F)
T3. @RCabreraBello -3 (F)
— PGA TOUR (@PGATOUR) June 18, 2021

It was Henley's sixth career score of 67 or better in a major championship and first since the 2018 US PGA Championship (65 in round two).

Molinari and Cabrera Bello remain a stroke off the pace heading into Friday, with the first round scheduled to resume at 06:50 local time.

Two-time U.S. Open champion Brooks Koepka is not far behind following his two-under-par 69 to kick off his pursuit of a fifth major crown.

Koepka, who finished second behind Mickelson at the PGA Championship, set the standard with four birdies in his first 11 holes taking him into a solo lead.

Russell Henley has company at the top of the leaderboard.@Louis57TM is tied for the lead at the #USOpen pic.twitter.com/GxWcOutbHx
— PGA TOUR (@PGATOUR) June 18, 2021

However, two bogeys meant he had to scramble to recover as Koepka ended the day alongside Schauffele, Hayden Buckley, Masters champion Hideki Matsuyama, John Rahm (through 17) and Sebastian Munoz (through 14).

Koepka has gone on to win or finish second in six of the last 10 majors which he opened with a score in the 60s.

World number one Dustin Johnson and star Rory McIlroy were both through 17 holes when play was called for the day.

Johnson had mixed a birdie with a bogey, while four-time major champion and 2011 U.S. Open winner McIlroy had an eventful start with three bogeys and four birdies.

Roars for Rors.@McIlroyRory is 2 back. pic.twitter.com/O3yWORko82
— PGA TOUR (@PGATOUR) June 18, 2021

Defending champion Bryson DeChambeau and his bid for back-to-back trophies started with a two-over-par 73.

American star Mickelson ended the round two shots worse off than DeChambeau following his forgettable 75.

Mickelson, who became the oldest major winner when he clinched the US PGA Championship last month, finished with five bogeys, including back-to-back on the front nine.