"I remember being really, really nervous on the first tee, which I'm not generally a nervous person but that was a new thing for me", Leishman said after carding a seven-under-par 64 in Lake Forest, Illinois. Four birdies on each side ensured Leishman finished the day atop the leaderboard despite the strong performances of Day and Fowler.
The most recent of those came at the Arnold Palmer Invitational this year.
"You have to be able to get connected to the hole (when putting) so that no matter how flawless the stroke is if you don't tie in the speed with the line that you've chosen and get it connected to the hole you're not going to make putts".
"It's nice when you can get on a run like this and hopefully I can keep it going", Leishman said.
Leishman was at 19-under 194 after the kind of round that limited the possibilities for the final day.
"It was a lucky hole-in-one, I guess".
Fowler started promisingly by sinking a 25-foot eagle putt from the fringe at the par-five first, but he struggled to buy a putt thereafter, his only birdie a near tap-in at the par-five 14th. You know, I've had two eagles and a hole-one-one this week already, so it's just... American Patrick Cantlay was alone in fourth after a 65 for 132.
Day still feels far away from 2015, when he won the PGA Championship and a pair of FedEx Cup playoff events.
Leishman has been the form Australian on Tour this year with a win and six top-10 results, but the 33-year-old Victorian says he'll draw inspiration from a memorable round in Chicago with Tiger Woods as he chases a third PGA Tour victory. His back was bothering him. But he came up short at TPC Boston two weeks ago - when he went into the final round sharing the lead with Justin Thomas and Jordan Spieth.
Reigning FedEx Cup champion Rory McIlroy showed signs of improvement with a 69 but the Northern Irishman remains well off the pace at one under.