By Mark Lamport-Stokes

OAKMONT, Pennsylvania (Reuters) - When the opening round of the weather-hit U.S. Open was finally completed on Friday, the unlikely leader was PGA Tour rookie Andrew Landry, who held a one-stroke advantage with several big names in hot pursuit.

Landry, competing in his first U.S. Open, had set the pace earlier on an overcast morning as he birdied his final hole for a four-under-par 66 at the rain-softened Oakmont Country Club.

American Dustin Johnson and England's former world number one Lee Westwood, each seeking a first major title, carded 67s while Spaniard Sergio Garcia, winner of the AT&T Byron Nelson in his most recent start, was among a group of three players on 68.

Among other golfing heavyweights in the field, defending champion Jordan Spieth opened with a 72, world number one Jason Day spluttered his way to a 76 and 2011 winner Rory McIlroy struggled to a 77 that included eight bogeys.

"Overall, I felt I played well," second-ranked Spieth said after mixing two birdies with four bogeys on a challenging Oakmont layout where only 11 players in the field of 156 broke par despite the more receptive conditions.

"I felt like I didn't quite get rewarded with my score for how I felt like I played. A couple tough breaks. It's a U.S. Open. I'm still in it."

Landry's opening round was the lowest ever in a U.S. Open played at Oakmont, eclipsing the 67s shot by Ben Hogan in 1953 and Gary Player in 1973.

"Just making putts," Landry said when asked what had been the key to his impressive start. "Game all-around was sharp, from tee to green to chipping to everything you do.

"I think the U.S. Open just suits my game so well that I'm just able to manage these things because I'm not a guy that's going to go out and shoot 60 and 61 and 62.

"I'm just a consistent guy that's going to shoot 68 and make a lot of pars."

SECTIONAL QUALIFYING SPOT

Landry, who earned his spot at Oakmont via a sectional qualifier, said he had spent very little time thinking about his birdie putt overnight before returning to the course and sinking a 10-footer to conclude his round.

"I just kind of thought about it this morning a little bit," he smiled. "I read the putt yesterday. I knew what it was doing."

Johnson, who tied for second in last year's U.S. Open at Chambers Bay one stroke behind Spieth after three-putting the final hole, moved into contention for his first major title by carding three birdies in a bogey-free display.

Shortly after completing his 67, Johnson headed back out to start the second round.

Masters champion Danny Willett returned a 75 while fan favorite Phil Mickelson, a U.S. Open runner-up a record six times without ever winning the title, shot 74.

With only nine players completing the opening round on Thursday, organizers hope to get the second round finished by early afternoon on Saturday.

Weather conditions look favorable for the weekend.

(Editing by Clare Fallon/Andrew Both)