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. 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". More...