While his veteran compatriot Adam Scott was doomed to an early exit after three double bogeys in his worst Open round in more than two decades, Leishman rose at 4am, teed off at 6.35am and enjoyed firing a second-round, three-under-par 68 on Friday.
The 41-year-old Victorian had bemoaned the snail-like crawl during an interminable first round in Northern Ireland, complaining he felt like he'd been on the Dunluce links "for about 12 hours" amid constant snarl-ups through slow play.
But freed up among Friday's very first group out, the LIV Golf veteran, who'd been two-over-par overnight, enjoyed a dazzling run of five birdies in the first dozen holes to move momentarily to within a shot of the lead.
Two bogeys at the next two holes, including a horrid shank off the tee at 13, checked his magisterial progress but he still became the first Australian to shoot in the red at this 153rd Open.
A decade on from his 2015 St Andrews near-miss when beaten in a playoff, Leishman was one under for the tournament, up into the top 20, having shot up over 50 places from his overnight position.
And he ended much cheerier than on Thursday when he'd finished with a bogey.
"I was obviously pretty frustrated last night when I came in here," he told reporters.
"I always say to my wife, 'if I've had a bad day on the course, just give me 10 minutes, and I'll be good' ... I think you got me about the eight-minute mark!"
Having got up at 4am - "I set the alarm as late as possible, I love my sleep," he sighed - Leishman could at least enjoy an afternoon treat with conditions forecast to be trickier.
"Happy to be watching it on TV. I'll probably have a pint or two of Guinness and enjoy my afternoon," he smiled.
On a good morning for scoring, it was American 2023 champion Brian Harman setting the early pace, shooting a tournament-best six-under 65 to move into a three-shot clubhouse lead at eight under over Britons Tyrrell Hatton (69) and Bob MacIntyre (66) and Dane Rasmus Hojgaard (68).
Out on the course, China's Li Haotong was just a shot adrift of Harman at seven under after seven holes.
But while the swiftly changeable conditions on the County Antrim coast were at their mildest with 10-15mph winds, Portrush still proved fiendish for Scott, who'll miss the cut in his 25th successive British Open and 97th straight major appearance.
Having started out as joint-leading Aussie alongside Cam Smith, after a first-round 72, Scott had been hoping to celebrate his eve-of-tournament 45th birthday with another stirring crack at winning the Claret Jug.
But as soon as he made double-bogey on the short third, having struggled to extricate himself from the wet rough, he was on the back foot. He dropped four more shots in three holes around the turn, including another double-bogey six at 11 after driving into some bushes.
His misery was compounded with a third double at the par-3 16th and a bogey at the last as he finished with an eight-over 79 - his worst round at the British Open since his 82 at Royal St George's 22 years ago - and a tournament total of nine over.
Midway through the afternoon, it was still possible Leishman might be the only Australian to make the weekend's action, with Elvis Smylie (three over after a 70), Min Woo Lee (five over, 73) and Cameron Smith (seven over after 10) among others set to miss the cut.
