Day was three-up after 14 holes when he winced in pain hitting a tee shot on the 15th hole, straining his lower back and suffering shooting pain.
He was able to scratch out halves in the next two holes to win the match but was in serious discomfort, struggling to walk.
“It’s really hard to play with searing pain in both sides of your lower back,” Day said between the 15th and 16th holes.
When McDowell missed a putt on the 16th to continue the match Day was filled with relief, but also serious pain and hustled straight inside for treatment.
He emerged about an hour later with painkillers and while still battling to walk freely was more optimistic.
“I’m struggling but I feel a lot better than I did an hour ago,” he said as he was ushered away from the course.
Day couldn’t confirm or deny at the time whether he would front up for his second match against Thongchai Jaidee on Thursday afternoon.
He could effectively concede the match and still return on Friday for his match against Paul Casey, hoping to advance to the knockout stage.
But with the Masters just two weeks away the injury couldn’t come at a worse time for the 28-year-old, who has battled injury and illness throughout his career.
Over the past five years he has missed tournaments through back, ankle, wrist and thumb injuries as well as others with illness and vertigo.
Before the injury he showed guts to come back from two down after the first four holes against the former US Open champion from Northern Ireland.
Marc Leishman bogeyed the final hole and had to settle for a halved match with American Ryan Moore while Matt Jones is one-down through 14 holes in his battle with South African former British Open winner Louis Oosthuizen.
Adam Scott has set up a two-up lead through 10 holes against Thomas Pieters while Marcus Fraser is one-down through eight to current British Open champion Zach Johnson.
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