(Reuters) - Jason Day was among the winners in Wednesday's opening day of the WGC-Dell Match Play, but the Australian world number two left the Austin Country Club course clutching his back heading for treatment. PGA Championship winner Day defeated former U.S. Open champion Graeme McDowell of Northern Ireland 3&2, catching fire with birdies on four of five holes from the fifth, but late in the round the world number two's back tightened up. The world number two declined to stop and chat with reporters but his agent later said Day expected to play his next match on Thursday. More...