In the 2013 U.S. Open, Horschel held the joint lead with Phil Mickelson, at the halfway stage of the tournament. Playing in his first major as a professional, he was one of only two players under par after the second round, hitting all 18 of Merion's greens in regulation. He shot rounds of 72–74 over the weekend to finish at five-over-par for the tournament and four strokes behind eventual winner Justin Rose. Horschel finished T4 in his first major as a professional and moved up to 34th in the world rankings and 4th in the FedEx Cup standings.