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Defending champion Shane Lowry is 12 shots off the lead held by William McGirt following the opening round of the WGC-Bridgestone Invitational at Firestone.
Coming off his best-ever finish in a major, Lowry mixed seven birdies and a birdie on his way to a six-over-par 76 to leave him in a share of 51st heading into the second round.
The Offaly native began the defence of his title with a bogey five at the opening hole but recovered the drop shot immediately as he birdied the par-five second. That was to be his only birdie of the round, however, as he bogeyed the fifth and sixth as he reached the turn in two-over.
Further bogeys followed at the 11th, 13th, 14th and 15th to leave him with a mountain to climb if he is to successfully defend his title.
Things were a lot easier for McGirt as he carded an opening 64 to leave him three clear of the field.
Less than a month after winning his first PGA Tour title at the Memorial Tournament, McGirt carded a bogey-free round, including two birdie putts from 30 feet and holing out from a bunker at the 15th.
"This course is like Muirfield. I love it, it fits my eye well - apart from the 18th, I can't work out how to hit the fairway," McGirt told PGA Tour Radio.
"I'm comfortable on older-style golf courses and I love the look of this one. I know I can compete out here and I know I can win it."
World number one Jason Day, Jimmy Walker and Emiliano Grillo are three strokes behind, while Rickie Fowler and Jordan Spieth are a further shot back on two-under.
Speaking after his opening round, Day told the Golf Channel: "I felt like I did all right. For the most part, I hit a lot of good quality shots, iron shots, into the greens and gave myself some opportunities out there.
"When I was kind of out of position, my short game saved me and I had some pretty nice up-and-downs to really keep the momentum going."
Spieth was two-over at the turn but rallied with five birdies on the back nine, including four successive birdies from the 15th, to finish just four strokes off the lead.
"I was making some nice par saves to grind it out and then finally those par saves turned into birdie putts," said Spieth.
"I won the battle in my head today. I really could have shot four or five-over pretty easily with the way that I was striking the ball tee to green."