ATLANTA — Since the United States Open in mid-June, Jason Day’s cumulative score in PGA Tour events is 101 under par. With a 61 here and a couple of 63s there, Day has turned what used to be regarded as a four-day marathon into a four-round sprint. Day, who came into the current Tour Championship with victories in four of his last six starts, has made the rest of his competitors feel like George Dole to Day’s Roger Bannister. Dole was one of five other runners in the field in 1954 when Bannister broke the four-minute barrier in the mile.