Russell Knox fired a 66 to set the clubhouse target on day one of the WGC-Bridgestone Invitational at Firestone Country Club.
The 32 year old became the first Scotsman to win a World Golf Championships event at the 2015 WGC-HSBC Champions and made an excellent start in his bid to add another as he got to four under.
Adam Scott was the leader on the course at five under after 11 holes, with Rory McIlroy and Thomas Pieters both alongside Knox playing the back nine.
Knox put his approach into his opening hole to ten feet and then holed a 19-footer to take advantage of the par five 16th. Grazed tree branches led to a bogey on the 18th but he birdied the first and then holed out from 75 yards on the par five next to jump to four under.
He made a smart up-and-down on the fourth, lipped out for birdie on the sixth and made a good save from the sand on the seventh to keep himself in that leading group. An excellent 27-foot putt on the eighth then moved him out in front on his own but he bogeyed the ninth to slip a shot behind Scott.
The Australian was absolutely dialled in with his irons as he birdied the first and second and when he holed from 30 and 26 feet on the sixth and eighth, he was four under. It was then back to smart iron play on the 11th as he holed from six feet for another birdie.
McIlroy holed from 25 feet on the 11th but gave the shot back on the 13th before a beautiful approach brought a birdie on the 18th. Back-to-back birdies on the second and third after more excellent approach play moved him further up the leaderboard before he needed a smart par save after a poor second on the fourth.
The 28 year old - who has best friend Harry Diamond on the bag after parting company with JP Fitzgerald - then rolled a 13-footer up the hill on the sixth to sit just one off the lead.
Pieters started with four pars but a stunning approach to two feet on the 14th got him into red figures before he put his third into the 16th to tap-in range. A putt from the fringe on the first brought another birdie and he made it back-to-back gains despite finding sand with his second shot on the next.
England's Ross Fisher and American Kevin Kisner were then in the clubhouse at three under, alongside Open Championship winner Jordan Spieth, World Number One Dustin Johnson, two-time Masters Tournament champion Bubba Watson and World Number Three Hideki Matsuyama, who were all still on course.
