Rory McIlroy carded a superb round of 66 to move within one shot of the lead at the season-ending Tour Championship.

The Northern Irishman began round one five shots off the lead as part of a staggered starting system derived from the PGA Tour’s FedEx Cup points list.

McIlroy, 30, raised concern with the scoring system but went on to shoot four birdies in a four under par round.

He moved to nine under par overall, a shot behind co-leaders Justin Thomas, Brooks Koepka and Xander Schauffele.

Only the top 30 on the PGA Tour’s FedEx Cup points list qualify for the Tour Championship and their starting scores work on a sliding scale down to the players in 26th to 30th, who begin on level par.

The eventual winner will pick up a prize of $15m (£12.2m) and McIlroy would have shared the overnight lead had he converted an eight-foot birdie putt on the last.

“I got off to a decent start. To play the back nine in three under and get in with a decent score is very nice,” he told Sky Sports.

“I would have liked to have birdied the last but I’m right there with three rounds to go. If I can replicate this round of golf three more times I think I’ll have a good chance.”

Thomas began the day at 10 under to hold a two-shot advantage on the field as a result of his standing in the FedEx Cup points list built throughout the season.

But the American only navigated Atlanta’s East Lake Golf Club course in level par – including a double bogey when he found water on the par-three 15th – to lose his advantage.

World number one Koepka was three under for his round but it was Schauffele’s bogey-free round of six under that caught the eye most to complete a trio of Americans at the top of the leaderboard.

England’s Paul Casey moved from his starting position at two under to end day one on six under par as three birdies on the back nine helped him to a share of eighth place.

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