Roberto Castro has recorded a first-round 65 to lead the BMW Championship at the end of Thursday's play.
However, many of the field are yet to complete their opening round after heavy rain in Indiana resulted in a rain delay which lasted several hours.
Castro - the world number 101 - recorded eight birdies and dropped just one shot as he opened up an advantage over American compatriot Brian Harman.
Major winners Jason Dufner and Dustin Johnson join Chris Kirk in a share of third position, while England's Paul Casey is among a group of four players who are three shots off the lead, although he still has eight holes to play.
Rory McIlroy is a shot further adrift although he too has eight holes remaining, but world number one Jason Day finds himself in 51st place after a one-over-par round of 73.
