Ardmore, Pennsylvania: Phil Mickelson shrugged off an overnight flight scramble and an early morning thunderstorm to post the clubhouse lead in the 113th US Open at Merion Golf Club on Thursday.
The American's three under 67, sandwiched around a 3 hours 32 minutes play suspension due to the bad weather, was still the target by the end of the day as half the field were unable to complete their rounds.
England's former world number one Luke Donald was best-placed among the chasers at four under through 13 holes.
Tiger Woods, meanwhile, appeared to be struggling with an injury to his left wrist after jarring it while playing out of Merion's thick and wet rough.
Mickelson skipped practise rounds at the course on Tuesday and Wednesday to fly back to California and attend his daughter Amanda's eighth-grade graduation ceremony, then flew back in time to start the Open on Thursday.
The airplane landed at Philadelphia at 4:15 in the morning and the 42-year-old left-hander reached the course 87 minutes later, well ahead of his 7:11 am tee time.
Seeking a first US Open title after finishing runner-up a record five times, Mickelson then set about tackling Merion's East course, hosting the US Open for the first time in 32 years.
That task he completed superbly, his sublime short game skills inspiring him to a 67, his lowest opening round in the US Open since 1999.
It gave him the luxury, as the clubhouse leader, of sitting back and watching the afternoon starters try to match him, knowing that, due to the delay, they would be unable to complete their rounds by nightfall.
Mickelson, who will turn 43 on Sunday, played down the difficulty of the flight logistics, saying that as a professional golfer he was accustomed to the situation.
"I got on the plane at 8:00, landed 3:30. Had a few hours sleep. We had a rain delay here, so I went and slept for an hour. I feel great," he said.
"So this is not that out of the ordinary; I do this about six, ten times a year where I fly back east red eye, play some outing and then come home. So it's not out of the ordinary."
With all of the 78 golfers who were in the 26 morning groupings having completed their rounds, big-hitting Belgian Nicolas Colsaerts was closest to Mickelson, two strokes back after a 69.
At level par 70 were South African pair Charl Schwartzel and Tim Clark along with Americans Rickie Fowler and Jerry Kelly who had a double-bogey six at the tough 18th, and Australia's Jason Day.
English pair Justin Rose and Ian Poulter as well as Americans Bubba Watson, Dustin Johnson and Steve Stricker were among those handily placed on 71.
There were early struggles for Sergio Garcia and Padraig Harrington, both of whom had 73s, while world number four Matt Kuchar disappointed with a 74.
The chase to match or better Mickelson's total by the 26 groupings that went out in the afternoon was further complicated by a second 45-minute holdup as another storm front swept through early evening.
By the time play was halted for the day, Donald led the hunt at four under through 13 with four players, including Masters champion Adam Scott, at two under with several holes yet to play.
Tournament favorite Woods, playing with the two men following him in the world rankings - Rory McIlroy and Scott - got off to an edgy start with two bogeys in the first three holes.
But he sunk a huge putt for birdie at the sixth to steady his round before jarring his wrist while hitting out of the rough at the 11th, minutes before play was called for the day.
Woods, whose last major win came at the US Open at Torrey Pines in 2008 when he won an 18-hole playoff against Rocco Mediate on what turned out to be a broken leg, is back near his best with four tournament wins this year already.
But to seal his comeback he needs to win a 15th major title and move to within three of the all-time record of 18 set by Jack Nicklaus when he won the 1986 Masters.
McIlroy, still waiting for his 2013 season to take off after an equipment change that has proved more problematic than he expected, was at level par when they came off.
The first round will be completed early Friday with organisers hoping to then get back on schedule to make the halfway cut by the end of the day.