SAN FRANCISCO (REUTERS) - World number five Dustin Johnson said he went "back to basics" after an inconsistent spell in which he won the Travelers Championship in June, only to miss the cut three weeks later at the Memorial Tournament in Ohio.

Johnson joins a stacked field at the PGA Championship this week and said he has fine-tuned his game after the disappointing showing at Memorial and subsequent withdrawal from the 3M Open a week later due to back problems.

"I was swinging terribly. My back was bothering me just from swinging back. I didn't hurt it doing anything. I hurt it swinging just because I was swinging so poorly," Johnson told reporters on Wednesday (Aug 5) in San Francisco.

The 36-year-old American finished 12th in Memphis on Sunday, after a handful of costly mistakes.

"Iron play was just not quite as sharp as I would like it to have been," he said.

"But 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, and I've got a lot of confidence in the iron game right now."

Johnson, who won the US Open in 2016 and finished runner-up at the 2019 Masters and PGA Championship, will attempt at TPC Harding Park to secure a second major title.

"After you win the first one, you also - you know you can do it so it definitely makes it easier, but it's never easy to win a major," he said.

"Every part of your game needs to be on point, and that's the difficult part about winning a major."