PGA TOURBMW CHAMPIONSHIPSite: Lake Forest, Ill. And in a jarring move, he made a decision to use an old school friend, Luke Reardon, as his caddie to replace the only caddie he ever had as pro - Colin Swatton, who doubles as his coach.
Their relationship dates back 17 years and the caddie aspect 11.
McIlroy won the Tour Championship past year to scoop the lucrative FedEx Cup bonus, but world number two Jordan Spieth leads the rankings.
Day, who last won 16 months ago, chipped in from behind the 14th green for his second eagle of the week, and then added a third eagle with one swing, which was actually a hole-in-one on the Par-3 17th hole with a 7-iron that turned into a payoff for multiple parties.
The US Women's Open champion mixed seven birdies with an eagle and a bogey for an eight-under 63 to take a two-shot lead over Thailand's Moriya Jutanugarn.
Leishman, seventh in the FedEx Cup rankings before the tournament, dropped only one shot for a second successive day. He's hitting it in the right spots, hitting it on the greens and holing putts. "That's a good formula for success".
Leishman's compatriot Jason Day and American Rickie Fowler are now in a tie for second-place on 14-under, after they carded respective rounds of 70, while England's Justin Rose has moved into contention on 12-under following a bogey-free score of 66. He is happy to be in the mix during what has been an emotional week, with constant questions about the change on his bag.
"Leishman is playing great, Rickie and Jason", said Rose.
Luke Reardon, a high school friend and accomplished golfer in their earlier days, got the assignment.
That ace carried Day to a 65, putting him in the last group on the weekend with Leishman. "Hopefully I can just play well, do the right things and not worry about the result too much, as hard as that will be".
That alone could have been a problem. "That's more conducive to good golf". But getting past that, Day said there is more of a comfort level than might have been expected.
"I don't want to be out there trying not to make bogeys", Leishman said. I felt like I hit decent shots, just didn't play well enough. I missed those today and, unfortunately, shot 1 under. We worked well together and hope we can keep that going. He has put himself in contention and is looking for a victory that would make this year look quite different.
Day, however, knows that overcoming Leishman could be a tall order on Sunday.
Marc Leishman remains the man to catch in the BMW Championship after another fine display saw him remain clear of his rivals both on the course and in Sky Bet's market where he is the 15/8 market leader.
"Bit of a grind out there today", Day said after his round.