By Mark Lamport-Stokes UNIVERSITY PLACE, Washington, June 15 (Reuters) - Two years ago, Hunter Mahan went into the final round of the U.S. Open at Merion just one stroke off the pace and held a share of the lead with four holes to go before falling back into a tie for fourth. Mahan signed off with a five-over-par 75 in difficult scoring conditions, but said later that he was happy with how he swung the club, maintained composure and stayed in contention for the title on what he described as "a brutal" day. "The mental grind of playing a major championship, especially a U.S. Open, is really the test," American Mahan told Reuters about his preparations for the second of the season's four majors. More...