BLAINE, MINNESOTA (REUTERS) - 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 (July 23).

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.

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.