McIlroy, Tiger Woods, Phil Mickelson, Brandt Snedeker and Nick Watney came into the final week of the FedEx Cup play-offs knowing the staggering payout would be theirs if they won the Tour Championship.
And it was Snedeker who emerged as the most likely to do so on Saturday with a third round of 64 to share the lead on eight under par with England's Justin Rose.
American Ryan Moore (65) was alone in third on six under, with McIlroy sharing fourth place on five under with Masters champion Bubba Watson and Jim Furyk.
McIlroy's 68 was his 11th successive sub-70 score, but the 23-year-old world number one knows he could need even better on Sunday to deny Snedeker.
The Northern Irishman, who carded two bogeys and four birdies, admitted saving par on the 18th from a greenside bunker could prove crucial, adding: "It depends on how I play tomorrow [Sunday].
"I wanted to make it (the par putt) because I didn't want to give a shot away right at the end."
He added on Sky Sports 3: "I felt like I played pretty solid but you have to stay patient.
"It's one of those courses where I don't think anyone is really going that low. I might need one and hopefully I have saved it for the last day."
Furyk, who lifted the huge prize two years ago on the eve of the last Ryder Cup, moved out in front on Friday by going to the turn in a spectacular 29 and handing in a 64.
But after one birdie and 15 pars left him in a share of the lead on the 17th, the Ryder Cup player ran up a triple-bogey seven after finding water off the tee.
Woods is four off the lead on four under after a 67, with Luke Donald a shot further back after matching that score largely thanks to holing his second shot to the 14th for an eagle two.
Snedeker had gone into the tournament aiming to be in contention to win on the final day.
He told the PGA Tour website: "My first goal for this week is achieved, and that is to have a chance going into Sunday.
"I think something like 90% of the PGA Tour winners come from the last two groups, and that's where I want to be - in those last two groups.
"So I now get I get a chance to do that tomorrow, and it should be a lot of fun.'' Rose praised Snedeker for recovering from a slow start to the tournament.
"I know he got off to a rough start on Thursday, and to climb back and tie for the lead being in that sort of vital top five is a wonderful performance," the Englishman said.
"So he's going to be the one out of the two of us thinking about it overnight.'' Rose plans to just focus on the Tour Championship win rather than worry about the FedEx Cup.
He said: "The whole year we've been grinding for the FedEx Cup, and it's still possible.
"It's not an easy golf course. Things can happen out there. But for me, it's about one tournament.
"I wish I would be in the situation that Brandt's in. I would trade him, but obviously, that's the position I'm in."