Thunderstorms rolled through the area in the afternoon, stopping play at 4:43 p.m. EDT. The delay lasted until 6:55, and action was later suspended for the day at 7:40. The first round will resume Friday at 7:00 a.m.
Angel Cabrera is also 4 under par through 12 holes in his round.
Haas made his U.S. Open debut in 1974 at Winged Foot. He competed at Shinnecock and missed the cut in 1986, but made an impressive return in 1995 with a tie for fourth. Now, back on the eastern tip of Long Island in 2004 and already on his way to the Champions Tour, this might prove to be his best effort yet.
'People asked me if this is the best I've played,' said Haas, a nine-time winner on the PGA Tour whose last title came at the 1993 Texas Open. 'I'm hitting the ball longer than I ever have. I feel more confident with my putting, my chipping, my short game is better because of my putting, I believe. But until I win, I won't say it's the best I've played.'
Haas got off to a quick start on the opening hole and hit a sand wedge to 3 feet for a birdie. He stumbled to a bogey at the second, but recovered at the par-4 fourth after his approach stopped within 5 feet of the cup.
The 50-year-old added a birdie at the fifth and ran home a 12-foot putt for a birdie at the par-3 11th. At the par-3 17th, Haas held the green, but had a lengthy effort left for birdie. He drained the long putt to reach 4 under and match Maruyama in the lead.
Haas cited the records of the past champions at Shinnecock, his playing partner Raymond Floyd, who won in 1986, and Corey Pavin, who titled in 1995, as examples of the type of game needed to find success at Shinnecock.
'You look at the two winners, Corey and Raymond, wonderful shot makers and wonderful short games, and I think that - not that I put myself in that category, but I think it allows most everyone here, length-wise, to be in the hunt,' said Haas, who also shot a 66 in the second round of the 1985 U.S. Open at Oakland Hills.
Maruyama had his best U.S. Open finish two years ago at Bethpage. After missing the cut last year at Olympia Fields, Maruyama returned to New York and found it to his liking.
'I just can't explain how happy,' said Maruyama. 'I'm very happy.'
Maruyama shared the stage with Tiger Woods in the opening round and was flawless with a birdie at the fifth and back-to-back birdies from the 10th.
At the par-5 16th, Maruyama chipped his third shot to 10 feet and converted for birdie to take the lead at 4 under. He parred his way in to join Haas in the early lead.
'I'm just trying to make a cut now, and I don't have much experience in that position in a major tournament,' said Maruyama, who is making his fourth U.S. Open appearance. 'I'll try to play my golf the rest of the three days, see how my golf works in a major.'
While there were plenty of birdies early on at Shinnecock, not everyone was able to escape the difficulty of the layout. Woods, who played along with Maruyama and Chad Campbell, tallied a birdie at the fifth, but gave that shot back with a bogey at the unforgiving par-3 seventh.
At the ninth, Woods left his second shot well short of the green in a mess of long grass. He flopped his third shot to 10 feet, but was unable to save par. Woods missed the green again at the par-3 11th, but this time the 28-year-old was able to get up and down.
Woods could not find the fairway off the tee at the 14th and sent his approach from the rough into a greenside bunker en route to another bogey. Woods was in trouble again at the very next hole, but hit a remarkable shot from just off the green to 3 feet and was able to save par.
The two-time U.S. Open champion then played his third shot to 7 feet at the par-5 16th but failed to make birdie on his way to a first-round 72.
'There's an awful long way to go,' said Woods, who withdrew as an amateur in the first round of the 1995 Open at Shinnecock after injuring his wrist. 'We haven't seen the wind up yet. If that ever happens, if it ever comes up, this golf course is pretty tough.'
Defending champion Jim Furyk, who was not expected to play this week, but is competing as his recovery from wrist surgery continues, was on fire early with birdies on each of his first two holes. Shinnecock took its toll, though, and Furyk bogeyed three holes on his way out to make the turn at 1 over.
Furyk countered with a birdie at the first, his 10th, but struggled again with a pair of bogeys starting at the third. He parred his way in for a 72 of his own.
'Obviously, I hit it in the rough my share, so the wrist felt good. It actually felt better today than it has all week,' said Furyk. 'I mean I am assuming that's probably a little adrenaline because I'm excited about playing.'
The morning groups did get a different look at Shinnecock and those teeing off in the afternoon were greeted with increasing wind, clouds and an oncoming storm. Pavin looked to rekindle the magic that earned him his lone major title and moved to 2 under with a birdie at the first, his 10th.
Phil Mickelson quietly made his way around the back nine with a birdie at the 12th and continued his par streak around the turn. Meanwhile, Cabrera was lighting up the front side with four birdies.
Then the rain came.
After players finally returned to the course, Cabrera parred his next few holes to stand at 4 under through 12. Mickelson picked up a birdie at the fifth to reach 2-under through 15 holes along with Vijay Singh, who is 2 under through 14. Pavin meanwhile bogeyed the second and stands at minus-1 through 13.
Reigning British Open champion Ben Curtis, Kris Cox, Jeff Maggert, Skip Kendall, Kevin Stadler and David Roesch are in the clubhouse at 2-under-par 68.
Other notables include 2003 Masters champion Mike Weir, who is in at 1-under-par 69, and two-time U.S. Open champion Ernie Els, who completed a round of 70 before the delay. Sergio Garcia, who won the Buick Classic last week, was in the group at 2-over-par 72.