By Mark Lamport-Stokes

OAKMONT, Pennsylvania (Reuters) - This week's U.S. Open is being played at Oakmont Country Club on the outskirts of 'Steel City' Pittsburgh where, appropriately, nerves of steel will be required to tackle one of the world's toughest courses.

The iconic par-70 design by Henry Fownes has long been recognized as the most challenging layout in North America with its lightning-fast, heavily contoured greens, deep bunkering, thick rough and very few flat lies on the fairways.

Phil Mickelson, still searching for his first U.S. Open title after a record six runner-up spots in the championship, described it as "one of the most difficult golf courses" while Jordan Spieth said it was "the hardest test in all of golf".

Though rain has been forecast for Thursday's opening round, which would soften a layout running firm and fast during the tournament build-up, Northern Ireland's Rory McIlroy did not expect that to be a significantly easing factor for the players.

"I'd say between having a little bit of rain in this forecast and not having rain might be a half-a-shot to a shot difference out here," the world number three told reporters on Wednesday.

"I really don't see it being that much of a factor. Rain or no rain on this golf course, it's still a very, very hard test of golf. It might soften it up a little bit, but it doesn't mean the scores are going to necessarily be that much lower.

"The balls might stay in the fairways a little bit more, and might hold on the green a little bit more, but it's still not going to make much of a difference."

OVER-PAR WINNING TOTAL

The last time the U.S. Open was staged at Oakmont in 2007, Argentina's Angel Cabrera triumphed with a winning total of five-over par and the game's best players are expecting something similar this week.

"I'd sign for even par," defending champion and world number two Spieth told reporters. "I don't think anyone's going to be in the red (under par) come 72 holes. So it will be a challenge, but I'm certainly looking forward to it."

Spieth, who clinched last year's U.S. Open by one shot at Chambers Bay to land his second major title, is known for his brilliant putting and he will be paying special attention to the slick greens this week.

"You can't let your mind slip on these greens for one moment, or else you're going to be left with possibly a 10- to 15-footer on the next putt, if not worse," the 22-year-old said.

"You really have to be cautious of it. I think they putt very similar to Augusta (National) and, in some places, they are more severe."

Mickelson, a five-times major champion, felt that Oakmont accomplished the goal desired by its members to have the hardest course in the world, or at least in the United States.

"There's no reprieve off the tee, there's no reprieve into the greens, and there's certainly no reprieve on the greens," Mickelson said.

"It is a very strategic course where you'll have to put the ball in the correct spots, and if you don't, kind of accept the fact you'll make a bogey and really try to minimize any doubles.

"If you play smart, you can probably minimize or eliminate doubles. If you do that, you should have a pretty good week."

(Editing by Frank Pingue)