By Mark Lamport-Stokes

OAKMONT, Pennsylvania (Reuters) - As a former Masters champion, Charl Schwartzel has conquered some of the toughest greens in golf but the South African rates this week's U.S. Open venue as an even more brutal test.

While the putting surfaces at Augusta National, permanent home of the Masters, are renowned for their heavy contours and lightning-fast pace, Schwartzel says there are "many more variables" on the notorious greens at Oakmont Country Club.

"These greens are more difficult than at the Masters," the world number 22 told Reuters after playing a practice round at par-70 Oakmont with fellow South Africans Louis Oosthuizen and Branden Grace on a sun-splashed Monday.

"Augusta has big slopes but they are gradual, almost constant in a way. Here you've got big slopes that have four breaks. The ball comes off a break and it will turn the other way and it will go back the other way and then down a hill.

"There are so many more variables here, so many slopes. To get speed right here is going to be so hard, and it's going to be key this week."

Making the challenge even more daunting for the players this week is the tangly Oakmont rough, some of the deepest Schwartzel has ever seen.

"This rough is really, really, really juicy," said the 31-year-old, who clinched his first major title at the 2011 Masters with a grandstand finish when he birdied the last four holes.

"The ball goes so down in this, you can get some lies where you can't even see the ball when you stand over it. It's just so, so, so severe."

EMBRACING THE CHALLENGE

As difficult as Oakmont will be for a field of 156 players gathered for the year's second major, Schwartzel says he will embrace the challenge.

"I like golf courses set up this way more than the ones where the guys can basically hit it anywhere and it becomes a little bit of a putting contest," said Schwartzel, who won his second PGA Tour title at the Valspar Championship in March.

"This really, really shows the weakness in your game if there is anything, and it even shows the weakness in your mind, if there is anything like that too.

"I love the way you have to prepare for these events, not just on your game and mapping the course out, looking at it from different angles but also how you set goals for your mind-set for the week. It's very different from what we normally play."

Schwartzel, who tied for 30th when the U.S. Open was last held at Oakmont in 2007, is excited about his form coming into this week after a 2016 campaign highlighted by wins on both the PGA Tour and the European Tour.

"I've had a really good year," said the South African, who landed his 11th European Tour victory at the Tshwane Open in his homeland in February.

"I feel like my game is in a lot better shape than it has been in the last few years. The last few weeks, I have played better every week.

"I made the most birdies at the Memorial out of everyone, I had 26 birdies," said Schwartzel, referring to the Jack Nicklaus-hosted Memorial tournament earlier this month where he tied for 11th. "I feel very comfortable."

(Editing by Andrew Both)