Rory McIlroy chips onto the green on the 15th hole during a practice round for the 2016 U.S. Open at Oakmont Country Club in Oakmont, Pa. on Wednesday, June 15, 2016. (Copyright USGA/Jeff Haynes)

Rory McIlroy says it will be a mental grind but Phil Mickelson is praying for the US Open will descend into utter mayhem.

Six times a runner up, the man they call Lefty was asked to describe the challenge of a course and said it was the toughest he’s ever played.

But the 46-year old, five-time major winner is also convinced that if he’s to complete the career Grand Slam and list the only major that’s missing from his CV, he needs conditions to be super-extreme.

He said: “I think that it accomplishes the goal that the members want, which is to have the hardest course in the world or in America or wherever, and I think they've accomplished that.

“I think that there's no reprieve off the tee, there's no reprieve into the greens, and there's certainly no reprieve on the greens.

“These greens are way more difficult to putt than Augusta's because where the hole locations are, they're pitched twice as much and the green speeds are comparable.

“Now, with all that being said, I believe it also gives me the best chance because after 25 years, you have to really know how to play this style of golf.

“It's just not like a regular Tour event. It's not like going out and playing golf at any other golf course.

“This is a whole different style of golf, something that over the years I've become very effective at playing.”

Rory McIlroy walks onto the green on the 15th hole during a practice round for the 2016 U.S. Open at Oakmont Country Club in Oakmont, Pa. on Wednesday, June 15, 2016. (Copyright USGA/Jeff Haynes)

With greens more slippery than the Pittsburgh Penguins’ ice rink, 200 cavernous bunkers and a strain of super-fertilised, four and a half inch heavy rough that makes you feel like your walking in superglue, the players are praying that today’s rain takes the sting out of the course.

“I would love to see it cross, the line the way US Opens often do, and become a little bit over the edge,” Mickelson said.

“That actually benefits me because we're going to have a winner at the end of the week. Whatever that score is, who cares if it's 5 under or 12 over, doesn't matter, the lowest score wins.

“So I would like to see it go over that edge because I feel like I've learned how to play that style of golf, and this golf course, specifically, even though past performance hasn't been it.”

McIlroy plans to play the course conservatively as he chases down title favourite and world No 1 Jason Day and defending champion Jordan Spieth.

And he’s hoping he has the patience to ride out the tough moments and shoot a score that keeps him in touch at a venue where five over par was good enough for Angel Cabrera to win in 2007.

Rory said: "You just have to be so disciplined. I'm an aggressive player as well. So there's just going to be times where I'm going to have to rein it back a little bit.”

Mickelson admitted that losing out to Justin Rose at Merion in 2013 was the biggest disappointment of his career and added that he thinks about completing that career Grand Slam all the time.

Member of the grounds crew cutting the hole on the practice green during a practice round for the 2016 U.S. Open at Oakmont Country Club in Oakmont, Pa. on Wednesday, June 15, 2016. (Copyright USGA/Joel Kowsky)

He said: “I could BS you and tell you I don't think about it. But no, I think about it all the time.

“This is the tournament I want to win the most to complete the four Majors. There's no question.

“There is nothing that would mean more to me than to cap off my career with a win here at the U.S. Open.

“Something that I've come close six times and that I've played well in the past but never have had that elusive win, where I've been able to capture the other three. It's my National open. It would mean the world to me.”

McIlroy succeeded Graeme McDowell as US Open champion in 2011 and like Mickelson, the Portrush man believes that there’s a need for attack and defence.

G-Mac said: “The key is knowing when to attack and when to defend and it’s the old major championship adage — you spend all day being defensive and you don’t make enough birdies.

“You have to be aggressive when you get the can and take pins on when you get the opportunity. You have to balance out the inevitable mistakes.

“I don’t have the power of some of guys, so I have to pick my battles out here.”

Shane Lowry has had such a tough year on the greens that he may need a good putting week to have any chance.

The good news is that he feels good about that aspect of his game, explaining: “I have a nice feeling about my putter this week and it's about committing to hitting good putts.”

What’s going to be more important is acceptance of bad breaks.

“It is one of those weeks where will be a lot of broken men leaving here on Sunday evening,” Lowry said. “And on Friday evening as well.”