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.
