The 22-year-old American won the first two majors of the year, contended in the other two, and has recorded two other victories.
That puts him well ahead of the field as the season enters its final four events, which culminate with the Tour Championship at East Lake with a $10 million (€13.6m) prize on offer to the eventual champion.
He has emerged as the true challenger to McIlroy, who has four majors to his name, and proved that by overtaking the Irishman as world number one after the US PGA Championship.
Spieth has every intention of keeping Holywood star McIlroy at bay and he knows the only way to do that is to keep on winning.
“The only way I think about going about it is just focusing on this week,” Spieth said ahead of The Barclays at Plainfield Country Club in New Jersey.
“If you win each week you’re going to stay number one and in my mind the goal is to approach each tournament to try to win and then try and keep this position for hopefully years.
“But again, it can change in two weeks’ time, so I’m aware of that. And that bit of fear on the back end of it is enough to get me going and to keep working hard.”
After making his major breakthrough this year, the season-ending finale, where money appears to be the greatest motivator, may seem something of an anti-climax for Spieth.
He admits it was not on his list of priorities and cannot possibly compare to the sport’s premier events but that does not mean he is not taking it seriously.
“I put winning the FedExCup below a major championship. I don’t think anybody holds it to the same level,” he added.
“I don’t know exactly where I put it. It’s something that obviously everybody wants to win, there’s no doubt about it, and it’s something I’d love to win some day.
“The names on that trophy are no fluke and so it’s something that hopefully I get a lot of chances at. This will be my third chance.”
Dustin Johnson withdrew from The Barclays pro-am midway through his round at Plainfield Country Club in New Jersey on Wednesday due to illness, the PGA Tour said at the opening event of the FedExCup playoffs.
Johnson, who missed an eagle putt for victory at the US Open in June, has been battling severe chest congestion since last week, a spokesman for the golfer said in a statement.
“After playing seven holes today, it became evident that he needed to leave the course and get as much rest as possible before tomorrow’s opening round,” the spokesman said.
“He is now under a doctor’s care, in hopes of a speedy recovery. Needless to say, he is doing everything possible to be ready for the important weeks ahead.”
Johnson was the second notable player to withdraw from the pro-am, following US PGA Championship winner Jason Day, who pulled out after feeling some back discomfort late on Tuesday.
The long-hitting Johnson, who won The Barclays when it was played at Plainfield in 2011, was replaced in the pro-am by Canadian David Hearn.
The Barclays launches the four-event, season-ending series that pays a $10 million bonus to the points leader following the Tour Championship at East Lake in Atlanta.