During the second round of the 2015 U.S. Open, Day collapsed on his 18th hole, the 9th hole on the course, having started the day on the 10th. He was very slow to get back up and was shaking and wobbly. It was later revealed that this was due to vertigo, a diagnosis Day had received from his doctor a month prior. However, the very next day, when he wasn't even sure if he would play, Day ended the third round tied for the lead. He finished the tournament tied for 9th. At the 2015 Open Championship at St Andrews, Day entered the final round as one of the 54-hole co-leaders and shot a bogey-free 70 to finish at 14-under-par and one stroke outside of the 3-way playoff. He had a putt for birdie on the 72nd hole to join the playoff but left it inches short. Day's tie for fourth, however, was his best finish at the Open Championship and was the sixth time he had finished in the top 5 of a major without winning one.