By Mark Lamport-Stokes

OAKMONT, Pennsylvania (Reuters) - Irishman Shane Lowry held a two-shot lead after 14 holes when third-round play was suspended for the day in fading light at the weather-hit U.S. Open after a marathon Saturday at Oakmont Country Club.

Lowry, showing nerves of steel on a challenging layout running fast and firm, mixed five birdies with two bogeys to get to five-under for the tournament before the horn sounded to halt the action at the 8.49 p.m..

PGA Tour rookie Andrew Landry, the surprise first-round leader, was at three-under after 13 holes, with American Dustin Johnson, after 13, England's Lee Westwood, after 15, and Spaniard Sergio Garcia, after 14, a further stroke back.

South African Branden Grace, who began the third round seven shots off the pace, was the leader in the clubhouse at one-under 209 after firing a sparkling 66 to surge into contention for the year's second major championship.

"The guys up there (at the top of the leaderboard) are playing some great golf," Grace told reporters after sinking a 15-foot birdie putt at the par-four last.

"I stayed patient the whole day ... but when I had the chances, I took them. It was a bonus finishing the way I did."

Just two strokes behind Grace was the ominous figure of Australian world number one Jason Day, who made an explosive start with four birdies in his first five holes on the way to a matching 66 that included an eagle at the par-five fourth.

On a gripping afternoon of blazing sunshine, five players held at least a share of the lead and there were frequent changes at the top as Oakmont lived up to its fearsome reputation for risk and reward.

The third round is scheduled to resume at 7 a.m. ET on Sunday before the regulation 72 holes are completed by the end of the day.

(Editing by Andrew Both)