Day fired a bogey-free five-under-par 67 to be tied for fifth midway through the first round of golf's $US25 million ($A38 million) unofficial fifth major at TPC Sawgrass.
The former world No.1's hot start followed three over-par rounds at last week's Arnold Palmer Invitational, where Day lost all confidence in his ball-striking.
He had posted back-to-back top-10 finishes before his joint-36th showing at Bay Hill so jumped at the chance of reuniting on the range with trusted mentor Chris Como.
Day credits Como with reshaping his swing to combat the debilitating back issues that threatened to end the former major winner's career.
"I hit it awful last week, and I didn't know what was going on," Day said on Friday.
"I lost a lot of confidence in my hitting. My targets were way too large. We had a two-way miss going on. When that happens, it's hard to commit to any type of shot."
But Como again ironed out the chinks to leave Day two shots behind co-clubhouse leaders Rory McIlroy and Xander Schauffele.
"I know he has different players as well as myself, so he has to kind of balance that out. But I haven't seen him (for weeks)," Day said.
"I typically don't like to work during the weeks that I'm playing, but I just tried to work on some of the technical stuff that we've been trying to get into over the last six months.Â
"That's difficult because I've got to teeter on the edge of being a little bit too sore in my back. I've got to really kind of find a good balance there. I feel like I found a nice balance.
"It was a little bit sore this morning, but then as the round progressed, it loosened up and it was totally fine."
Off to a strong start here at — Jason Day (@JDayGolf) @THEPLAYERS Ready to get back to work tomorrow 👊� pic.twitter.com/pRvyLXRFHoMarch 14, 2024
The 2016 Players champion spent several hours on Monday, Tuesday and Wednesday with Como and said the overtime paid special dividends.
"It's great. A bit of a dramatic difference between my hitting from last week to this week," Day said.
"Obviously it's only the first day, but felt like I did a lot of good quality work with Chris at the start of the week. Drove it nice.
"That's probably one of the most crucial things is to drive it well here. Even though it's not like overly long, getting it in the fairway is important.
"It was nice to be able to capitalise on the par-5s today, keep the fives off the scorecard and birdie those."Â
Countryman Adam Scott, who won at TPC Sawgrass 20 years ago, also made a fine start with a two-under 70.
Min Woo Lee carded a 73, while Cam Davis opened with a disappointing seven-under 79 after sharing sixth with Lee last year.
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