England's Justin Rose has a two-shot lead at the end of day one at the BMW Championship at Cog Hill.
The world number 40 holed nine birdies and eight pars in an impressive opening round of 63 in Chicago.
Americans Webb Simpson and Mark Wilson are two shots behind Rose, while South Korean K.J. Choi is four shots behind the leader with a round of 67.
Fellow Englishmen Brian Davis and world number one Luke Donald finished with rounds of 74 and 75 respectively.
Starting on the back nine, Rose picked up shots at the 10th, 11th, 15th and 17th, with his solitary blemish coming on the par-four 13th.
"I certainly didn't expect that going out there today," Rose said.
"I looked to the weather, looked at the temperature and I thought today was going to be a day to hang in there. Little did I know I was about to play so well.
"It's certainly my best round of the year by a long, long way, and could have been top five, top 10 rounds I've ever played for sure. That was really, really good out there."
Simpson managed a round of 65 while Wilson's round included an eagle to leave the American duo two shots clear of Choi.
Davis and Donald both struggled to find form while Scotland's Martin Laird finished with a round of 72.
In addition to the $1.44m (£1.03m) top prize, the field of 70 in the third US PGA play-off tournament are aiming for a place in the top 30 play-off point spots to reach next week's Tour Championship.