Day, the 2017 winner of the PGA Tour's flagship $US25 million ($A35 million) event, carved out a two-under-par 72 to be three shots off the pace after the weather-hit first round.
Maverick McNealy, Lee Hodges, Sahith Theegala, Austin Smotherman and Austrian Sepp Straka share the lead at five under after rain forced a morning stoppage and late suspension of play at golf's unofficial fifth major championship.
While McNealy, Hodges, Theegala and Straka all posted 67s, darkness left Smotherman needing to return on Friday morning (Saturday AEDT) with a short birdie putt to reach six under and snare the solo first-round lead.
Two-time PGA Championship winner Justin Thomas is among four players in a tie for sixth just one stroke back on a cluttered leaderboard devoid of the sport's biggest names.
World No.1 Scottie Scheffler could only manage an even-par 72, while defending champion Rory McIlroy opened with a 74 to be tied for 69th after withdrawing from last week's Arnold Palmer Invitational with back spasms.
After missing two straight cuts since opening the year with a runner-up showing at The American Express, Day finally rediscovered some form a month out from the Masters.
Scrambling brilliantly, the former world No.1 conjured four birdies, his otherwise tidy round only tarnished by finding the water off the tee on the par-3 island hole.
In contrast, Adam Scott almost aced the 17th, only to pull his drive into the water at the last to rack up a double of his own in a round of 72.
The 45-year-old is making his 24th appearance at TPC Sawgrass, the most of any player in the field, and still seeking a second victory after saluting back in 2005.
Fellow Australian Min Woo Lee also carded a 72 despite opening the day with a double on the first hole.
Fresh off an impressive top 10 at Bay Hill, Lee dug deep with four birdies on the back nine to stay in the mix.
Karl Vilips also rallied late for a one-over 73, but Cam Davis's woes continued with a 77 as he battles to make the halfway cut.
New Zealander Ryan Fox pulled out before the start of play, instead spending the day in a hospital to have kidney stones removed.
Fox cited "illness" as the reason for his withdrawal and later posted to Instagram that this was hardly a case of the flu or a stomach virus.
"Not exactly the week I had planned," he wrote.
"Unfortunately some kidney stones took me down on Tuesday night and then had to be dealt with surgically this afternoon.
"Devastated to miss out on such an amazing event, hoping to have a quick recovery and be back at it soon."
