Rory McIlroy signed for a first-round 77 on Friday to sit 11 shots behind surprise leader Andrew Landry in the U.S. Open.

A hectic Friday was in prospect at Oakmont after thunderstorms limited the opening-day action and prevented half of the field from even teeing off.

The unheralded Landry had just one hole to finish when play resumed and holed a birdie putt at the ninth, his final hole, to complete a four-under 66 - the lowest first-round score in U.S. Opens at Oakmont.

However, four-time major winner McIlroy - the victor in this event at at Congressional five years ago - slipped further down the leaderboard courtesy of dropped shots on the last three holes.

Reigning champion Jordan Spieth, one over through 11 holes overnight, had to settle for a 72 - leaving him six off the lead but by no means out of contention.

Among those faring better on a rain-softened course were Lee Westwood, who climbed into second at three under with two closing birdies, and Shane Lowry - the benefactor of three back-nine birdies in his two-under 68.

Bubba Watson, Kevin Streelman and Danny Lee matched the 69 managed by amateur Scottie Scheffler on Thursday, while a handful of other players were under par in the early stages of their opening rounds.

Yet the likes of Jason Day, Phil Mickelson and Adam Scott were still to get under way.