World number four Dustin Johnson withdrew from the 3M Open in Blaine, Minnesota with a back injury after struggled badly during his first round on Thursday.
Johnson, who missed the cut at the Memorial Tournament last week after two rounds of 80, shot a seven-over-par 78 at TPC Twin Cities.
"It's just my iron play. I feel like I'm driving it well, but the iron play, first six or seven holes hit it close and then the rest of the day kind of struggled a little bit with iron play," Johnson said.
Waterford's Séamus Power is tied for 15th on three-under after an opening round of 68.
Power lies three shots off the lead jointly shared by the USA trio Tom Finau, Ryan Moore, Nick Watney and China's Xinjun Zhang.
Meanwhile, Johnson, the highest-ranked player in this week's field, picked up his 21st PGA Tour victory last month and did not tee it up again until last week in Ohio.
The long-hitting American started his day on the back nine and seemed fine after an early birdie.
Johnson followed a bogey at the par-four 16th with a double-bogey at the par-3 17th where his tee shot found the water. He made quadruple-bogey at the par-five 18th after hitting three consecutive balls into the water.
"It was going okay until I hit it - well, I just hit a poor shot on 17. I hit it a little heavy in the water, made five there," said Johnson.
"But then hit a great drive on 18 and we only had like 199 to cover from where I was, it was a perfect 6-iron. Hit it right at it and never once did I think it was going to go in the water.
"Just went in the water and I hit two more shots in the water, then I hit a good one, made a tap-in for a 9."
Johnson had previously committed to play in next week's World Golf Championships-FedEx St. Jude Invitational at TPC Southwind in Memphis.
Meanwhile, in the British Masters at Close House, Co Down native Jonathan Caldwell is the highest placed Irishman after a second round of 69 to follow up Wednesday's round of 67 to leave him on six-under.
Wicklow's Paul Dunne is five shots behind Caldwell on one-under after carding a second round 70.
Italy's Renato Paratore produced a fine round of five-under-par 66 to lead at the halfway stage.
Cormac Sharvin missed the cut after a round of 70 left him on one-over, while Gavin Moynihan scored a second round of 76 to leave him on seven-over.