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.
