Yet on a day when heavy rain and worsening weather suggested the two major winners had enjoyed the kindest of the conditions at the County Antrim links, Smith's one-over 72 and Day's two-over 73 could still end up looking rosier come the end of Thursday.
Still, halfway through the opening day, there were 33 players in the under-par red figures, led by surprise frontrunners, world No.354 Jacob Skov Olesen and China's Li Haotong, as well as Britain's former US Open champion Matt Fitzpatrick, who all shot four-under 67s.
Even more ominously for the field, world No.1 Scottie Scheffler finished with a three-under 68, just a shot adrift.
So, both Smith, the 2022 Open champion, and Day, the 2015 PGA winner, are already playing catch-up. They both hit miserable drives off the opening tee, struggled to three-over across the first nine holes before at least 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."
Among the rest of the nine-strong Aussie contingent, Adam Scott, the day after celebrating his 45th birthday, was two over after 11 holes, Marc Leishman three over after 13 and Elvis Smylie three over 10.
