(Reuters) - Jason Day was among the early 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, near flawless with five birdies, but late in the round his back tightened up. McDowell was 2-up through four holes before Day, the 2014 winner of this event, caught fire with birdies on four of the next five holes to gain control of their group stage match. More...