Day, who is ranked second in the world and won his eighth PGA Tour title on Sunday at the Arnold Palmer Invitational, was cruising along on the back nine at Austin Country Club when he appeared to favor his lower back on the 15th hole. The pain was obvious when he hit his tee shot on the 16th hole, where he managed to make a par and close out McDowell 3 and 2.
"It's really hard to play when there is searing pain in both sides of your back,'' Day told The Australian Associated Press as he made his way to the clubhouse. "I'm really struggling. I need to get treatment right away to see what's wrong.''
Day's agent, Bud Martin, declined to let the golfer speak to reporters or a PGA Tour media official after getting treatment, then two hours later released a statement.
"Jason tweaked his back on hole No. 15 and had some work done post round,'' Martin said. "He is pleased with the outcome of the match and is getting prepared to play tomorrow.''
Because of the round-robin format, Day could concede his Thursday match to Thailand's Thongchai Jaidee and still compete Friday against Paul Casey. Jaidee, who defeated Casey on Wednesday, would be given a full point and would be in a strong position to advance to the final 16 this weekend.
Day is coming off a huge 2015 in which he won the PGA Championship and four other titles, including the Barclays in August, where he suffered from back issues that caused him to miss the pro-am. It is unclear if those back problems are related to what ailed him Wednesday. The Australian also withdrew from the Barclays in 2014 with a back injury.
Day, 28, has been plagued by injuries and medical issues in his career. Aside from the back woes, he also has withdrawn from tournaments due to bouts with vertigo as well as ankle, wrist and thumb injuries.