Despite having arrived at Merion Golf Club in the early hours of the morning on an overnight flight from his native California, Mickelson soared into contention at the year's second major with a three-under-par 67.
The American left-hander, who carried five wedges but no driver in his bag to tackle Merion's short East Course, mixed four birdies with a lone bogey after setting off from the 11th hole in the second group of the day.
Mickelson equalled his lowest ever opening round at the U.S. Open, having previously fired a 67 at the start of the 1999 championship at Pinehurst where he finished in second place.
"I loved having an early tee time," Mickelson told NBC. "I wish we didn't have the rain delay because it made for a little bit longer day, but the golf course is playing about as easy as it could and yet Merion is really fighting hard.
"We were having a hard time scoring low here. It's so demanding. It's one of the best (set-ups) I've seen for a U.S. Open. We had soft conditions and still we're all having a hard time getting under par."
Long-hitting Belgian Nicolas Colsaerts carded a 69 at Merion on Thursday while Americans Bubba Watson, Dustin Johnson and Steve Stricker, and English world number five Justin Rose, returned matching 71s.
However, the opening round was destined to spill over into Friday after the threat of lightning forced play to be suspended for just over three-and-a-half hours earlier in the day.
The siren sounded at 8.36 a.m. to halt the action under darkening skies, with Englishman Ian Poulter the early leader at three under par after recording birdies on his first three holes.
FELL BACK
When play eventually resumed, however, Poulter immediately fell back with a bogey at the par-four 14th, where his drive ended up in heavy, wet rough.
Four-times major champion Mickelson, runner-up at the U.S. Open a record five times, was at level par after five holes when play was suspended, and he mounted his charge after returning to the course.
The 42-year-old rolled in a curling 25-foot birdie putt at the par-four first, then did well to save par at the fifth where his tee shot nearly ended up in a stream.
He knocked in a short birdie putt at the seventh before claiming the outright lead by sinking a 30-footer at the par-three ninth.
What made Mickelson's round even more remarkable was the fact that he had returned to Merion in the early hours of the morning on his private jet from San Diego, where he had attended his oldest daughter's eighth grade graduation ceremony.
Tournament favourite and three-times champion Woods was among the late starters at Merion, scheduled to tee off with second-ranked Rory McIlroy and Masters champion Adam Scott in the most eye-catching trio of the day.
However, Woods and company may not complete that many holes on Thursday with severe thunderstorms expected to hit the Philadelphia area in the late afternoon.
