By Steve Keating NEW YORK, Aug 31 (Reuters) – American qualifier Ryan Harrison pulled off the biggest shock yet of the U.S. Open by beating fifth seed Milos Raonic 6-7(4) 7-5 7-5 6-1 on Wednesday to reach the third round of the year’s final grand slam. The 24-year-old American, who had beat Raonic once in their previous two meetings, came prepared to give the sixth-ranked Canadian a battle and proved the more durable during in a three hour, 37 minute match played under punishing conditions. While Harrison remained cool and collected it was Raonic, 25, who wilted and called for the trainers several times to deal with cramps and troublesome left wrist.