McIlroy fired a three-under-par 67, putting him in a tie with American's Jordan Spieth and Gary Woodland before play was stopped with nine players yet to complete their rounds at Cherry Hills Country Club just outside Denver.
It was a surprising top score after plenty of competitors, including McIlroy, had suggested earlier in the week that much lower numbers could be attainable at the third of the PGA Tour's four lucrative FedExCup playoff events.
Americans Billy Horschel, Russell Henley, Chesson Hadley, Kevin Chappell and Matt Every plus two-times major winner Martin Kaymer of Germany, Canadian Graham DeLaet and Spain's Sergio Garcia were part of a group of nine players one shot back.
Defending FedExCup champion Henrik Stenson of Sweden was also at two-under with a hole to play.
Henrik Stenson of Sweden (Reuters)
The 25-year-old McIlroy started on the back nine, claiming birdies on the 12th and 17th holes for a respectable 34 before he caught fire at the turn.
Birdies at the first, second and third holes catapulted him into the lead at five-under before he capped his round with consecutive bogeys.
"I'm a little frustrated coming off the course, because I feel like it should have been better than what I finished," said McIlroy. "A sloppy bogey or two out there.
"I felt like I hit a good drive off of seven, and I got into a bit of a tricky spot and made bogey there. And then to make a bogey straight after that, I was a bit frustrated with it."
With the short course affected by altitude many experts predicted low numbers but firm and fast greens ensured the historic old course held its own.
"It's tricky, it really is," said McIlroy. "The altitude, we have had a couple of days to adjust to that and it's fine but these greens have gotten so much firmer over the last 24 hours."
Spieth, the 21-year-old young gun, bettered McIlroy with six birdies but had three bogeys to find his place at the top.
He nailed two birdies in his final four holes in what he described as a stress-free round.
"Anything under par is a good score so I was really pleased," said Spieth. "My short game's never been as good as it was today."
The top 30 players on the FedExCup points list after the BMW Championship advance to next week's season-ending Tour Championship, where any of the top five would automatically clinch FedExCup honours and a staggering $10 million bonus with victory in Atlanta.
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