Dustin Johnson cited his bad back for failing to break 80 at the Memorial tournament last month in his first event since winning the Travelers Championship to extend his career win streak to 13 years.
The injury also caused him to withdraw from the US PGA Tour's 3M tournament before rebounding with a 12th-place finish on Sunday at the WGC St. Jude Invitational in Memphis.
"I was swinging terribly," Johnson said. "My back was bothering me just from swinging. I didn't hurt it doing anything. I hurt it swinging just because I was swinging so poorly.
"I rested for four days, got treatment, and then went out and practiced Monday at home. I just went back to the basics, worked on the right things and started hitting the ball well again.
"Going into Memphis, I was fine and confident. It was a golf course that I like and have played well on.
"It was really close to being really good last week. Felt like I played pretty solid."
Johnson is hoping his sore back doesn't flare up again at this week's PGA Championship, which will be played without spectators because of the coronavirus pandemic.
"This week I've put in a lot of good work, and I feel like the iron play is definitely back to where it should be," Johnson said.
He nearly chased down Brooks Koepka in the final round at last year's PGA Championship at Bethpage Black before losing by two shots.
"Sunday last year at the PGA was a good day. It was a tough day, but I was playing really well and kind of had it going there, especially coming down the stretch," he said.
Johnson, who is seeking his 22nd PGA title, won the Travelers in June to end a 15-month drought and stretch his career win streak to 13 years. Arnold Palmer and Jack Nicklaus each won in 17 consecutive years.
Johnson withdrew from the 3M after an opening 78. At The Memorial, he shot back-to-back 80s and finished 126th out of 128 golfers -- the worst 36-hole stretch of his PGA career.
Johnson said some sloppy play, and nothing to do with a swing change, led to the back problems.
"I just got into a bad habit," he said. "I was just hanging on my left side real bad and just aggravated my back, that's all."
Johnson, the 2016 US Open winner is playing the first two rounds in a group with Jordan Spieth and England's Justin Rose.
Johnson says in order to claim his second major championship, he needs to make every drive count.
"I've got to drive it the best," he said. "And obviously, always, you need to putt well. I think if I can drive it really well, I'll have a good chance to win."