Yet on a day when heavy rain and worsening weather around lunchtime suggested the two major winners might have had the kindest of the conditions at the Antrim links, Smith's one-over 72 and Day's two-over 73 could still end up looking much rosier by the end of Thursday.
The tournament's surprise early frontrunner was world No.354 Jacob Skov Olesen, a Danish left-hander who shot a four-under-par 67 to take the clubhouse lead alongside China's Li Haotong.
But both Smith, the 2022 Open champion, and Day, the 2015 PGA winner, found it tough going after hitting miserable drives off the opening tee, struggling to three-over across the first nine holes before rallying in pedigree fashion after the turn.
Smith smiled that he had shown a bit of "Queensland spirit" over the final nine holes, after his awful, scuffed 150-yard drive at the first had set an early dispiriting tone.
"I think I tried to hit that one underground - and almost missed it. Not the greatest start to a major championship I've ever had," he sighed.
A double-bogey at the ninth left him in strife, but he pulled back two shots to par on the homeward stretch, sinking a 30-footer at the short 16th to give him a real shot in the arm.
"I did a really good job of hanging in there," Smith said.
Day was not so happy about his effort.
"To work hard coming into this week and then to go out there and have poor process is not acceptable, especially in my eyes," said the 2023 Open's joint runner-up at Hoylake.
"I've got to do a better job of it and hopefully come back tomorrow and do it again, play better."
The in-form Lucas Herbert started brightly and was among the early leaders at two under, but four bogeys over the last eight holes left him with a three-over 74.
Ryan Peake's big adventure in his first major went somewhat awry as he ended with a six-over 77.
The Australian former motorcycle gang member who turned his life around through golf after spending time in prison for assault has been the centre of much attention at Portrush this week after gaining his place thanks to a fairytale win at the New Zealand Open.
Partnering former champion Phil Mickelson on Thursday, 31-year-old Peake admitted to being a bit star-struck and ended up getting the six-time major winner to sign his glove and give him his ball after the round.
"Lefty'' Mickelson conjured up some of his old magic in a one-under 70, the highlight being his holed bunker shot for par at the third hole after he'd left his first effort in the sand.
However, Peake struggled, admitting: "Very disappointed with the round.
"Not what I want, pretty flat at the moment. Was pretty flat out there as well. I just got beat up out there."
Some of the tournament's biggest names were enjoying encouraging starts, with world No.1 Scottie Scheffler at two under after 10, and the last Portrush champ Shane Lowry one under at the same hole.
Reigning champion Xander Schauffele, Jon Rahm and US Open champ JJ Spaun were all at level par after 11.
