The BMW Championship is the penultimate event of the PGA Tour season. He was 31st in that category entering the week.
Meanwhile, Leishman carded eight birdies in a seven-under 64 round on Friday to extend his lead heading into the weekend.
Justin Rose muscled his way into fourth place thanks to five birdies in a bogey-free 66 for a 12-under tally but admitted he has his work cut-out to reel-in the Aussie.
"It's slowly building, " said Day, who began this year at No. 1 in the world and has dropped to ninth. The Olympic gold medalist from England said he noticed a slight uptick in the wind on Saturday. The second-round stroke average was 69.072, while the first-round average was 68.855.
With Fowler and Day unable to apply significant pressure, Leishman reeled off a series of pars until he holed another good putt from mid-range at the 13th, and a deft up-and-down from the rear rough at the final hole yielded another birdie which saw him close on 19 under par.
FedExCup No. 5 Jon Rahm could do no wrong earlier this season, registering seven top-10s in a span of 10 starts, highlighted by his win at the Farmers Insurance Open. He's hitting it in the right spots, hitting it on the greens and holing putts.
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Day blitzed the field to win here in 2015, the last time the BMW came to Chicago's North Shore. "The course changed quite a lot today compared to the first two days, firmed up a lot and greens sped up so that was good to see and nice to keep making birdies like I have been all week".
'And last week I showed that a couple of shots going in the back nine is not enough. We've played pretty much every Wednesday and every Saturday together throughout our high school years, and being able to have those conversations, putting my 100 percent trust in his ability to be able to tell me, 'OK.
The top three players in the FedExCup standings, Americans Jordan Spieth, Justin Thomas and Dustin Johnson, also had late tee times.
"Knowing I had already made the BMW probably affected my concentration a little in the first two FedEx Cup play-offs events".
Acknowledging his hopes of retaining the FedEx Cup are all but over, out-of-form former world number one Rory McIlroy is relishing the opportunity to take a break and return recharged for 2018.
Get your popcorn and your cooled beverages at the ready, going to be a birdie fest tonight.