Day, the 2016 champion at Sawgrass, was 2-over when he withdrew on the par-5 16th – his 7th of the day – citing a back injury incurred while practising last week.
Spain’s Rafa Cabrera-Bello leads the tournament at the Bay Hill Club and Lodge in Orlando, his opening 7-under 65 leading by two from American Keegan Bradley.
Day, a winner at Bay Hill in 2016, had just pulled his second shot at the par-5 into the water when he told playing partners Rickie Fowler and Ian Poulter he was heading back to the clubhouse.
Quizzed by reporters after the round Day, who didn’t play a practise round or the pro-am this week, said he had hoped the injury would settle down during the round but it hadn’t.
"I couldn't play at 100 per cent today, so I just wanted to see if I could get out here and may have loosened up,” he told reporters after the round.
"But unfortunately it didn't, so I had to pull out."
Day didn’t say where he initially incurred the injury but said he had aggravated it last week. "I (aggravated) it last Sunday, and then got an MRI Monday which came back that I had an annular tear in my disc," Day said.
"My back was sore when I was practising from Tuesday to Saturday and I was going to practise on Sunday, but I woke up and couldn't really walk or sit in the car.
"I was on a dose pack to try get the inflammation out of it and that didn't get any better. I saw a physio here (in Orlando) and tried to do as much work as I possibly could to get ready for this week.”
With next week’s Players’ Championship one of the biggest tournaments of the year Day said he was hopeful he would recover in time to play.
"I think it's just rest, to be honest,” he said, “and hopefully I'll be ready next week."
Day was one of just three Australians to start the week, Aaron Baddeley the best of the remaining two after 18 holes with a 2-under 70 to be T15.
Marc Leishman impressed with a battling an even par 72, a remarkable effort after being 4-over through six holes.
Leishman turned in 40 but posted four back nine birdies, including a brilliant effort at the 18th, to salvage what would otherwise have been a disastrous day and head to Friday T49.
BEHIND the fence.
Phil's not afraid to play any shot. pic.twitter.com/tGizOzdBYy— PGA TOUR (@PGATOUR) March 7, 2019
Veteran Phil Mickelson is playing the tournament for the first time since 2013 and while his 4-under was a good score it was his antics on the 10th hole which stole the headlines.
Lefty drove his ball under a netting fence separating the course form a neighbouring backyard and was forced to turn over one of his irons and take a right-handed swipe.
It was a less than successful effort , the ball getting tangled in the net before dropping in almost the same place it started.
Mickelson then took a drop and eventually wracked up a double bogey but in typical lefty style still managed to shoot 68 and head to the second round T3.
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