By Mark Lamport-Stokes

OAKMONT, Pennsylvania (Reuters) - Long-hitting American Dustin Johnson, seeking major redemption after several near-misses in recent years, won the 116th U.S. Open on Sunday amid high drama and initial uncertainty over his victory margin.

In pursuit of his first grand slam crown, the world number six played clutch golf under intense pressure at brutally difficult Oakmont Country Club, finishing in style as he sank a five-foot birdie putt at the last.

Having negotiated the final seven holes under notice that he might be penalized for a rules infraction on the fifth green, despite being cleared by another official at the time, he was docked a shot after the round's completion, with his score amended to a closing 69 for a four-under winning total of 276.

After sinking his final putt, Johnson pumped his right fist in delight, and then lifted his putter to acknowledge the loud roars from the crowd before being embraced by his caddie.

"Feels good, feels really good, feels well deserved," an emotional Johnson told reporters after finishing three shots in front of fellow Americans Jim Furyk (66) and Scott Piercy (69), and Irishman Shane Lowry (76).

"I've had a lot of opportunities that I didn't quite get it done. So this one's definitely really sweet. I knew I was swinging well, and I just kept thinking, it's just me and the course.

"I'm playing against the course. I can't control what anyone else does. So just tried to hit golf shots, tried to hit it on the correct side of the hole, and two-putt."

The rules controversy was sparked when Johnson's ball moved slightly as the world number six was preparing to attempt his par putt at the fifth hole.

He denied having caused the movement and the official accompanying the pairing decided not to levy a penalty.

After being was informed on the 12th tee by a U.S. Golf Association official that he might be penalized after the round, Johnson ran up a three-putt bogey at the 14th but otherwise displayed nerves of steel as he negotiated the closing stretch.

Thunderous roars of "DJ, DJ, DJ" and "USA, USA, USA" rang out as the crowd gave vocal support to the tournament leader in his bid for a first major victory after a series of heartbreaks in the past.

American world number two Spieth, who won last year's title by one shot, finished a disappointing title defense with a 75 for a nine-over total of 289.

(Editing by Andrew Both)