Defending champion Justin Thomas says he can't take much from last year's victory into this year's PGA Championship, but he got a major boost from a round two weeks ago.

The 30-year-old American chases his third major title this week at Oak Hill, where a third PGA title would put him behind only five-time winner Jack Nicklaus and four-time winner Tiger Woods in the stroke play era (since 1958).

Thomas won the 2017 PGA and last year made the greatest last-day fightback in tournament history, rallying from seven strokes down to beat Will Zalatoris in a playoff at Southern Hills.

"It's so long ago in terms of your golf game," Thomas said Monday. "The feelings can translate and the memories can translate, but a lot of things – swings and putts and chips – have happened since then. That part is hard to relate."

Since that victory, Thomas has managed only three top-five finishes in 19 PGA starts. But he shared 14th two weeks ago at Quail Hollow, site of his 2017 major triumph, after a third-round 70 that he said revived his confidence in his shotmaking.

"I showed a lot of really good signs in Charlotte," Thomas said. "I was hitting a lot of very poor wedges and irons. I birdied two of the last four holes and salvaged an under par round on a tough golf course."

Thomas caddie Jim Mackay, nicknamed "Bones", was thrilled at the effort.

"Bones and I said on 18 green, this is the stuff that we haven't been doing this year," Thomas said. "That 70 then gave me an opportunity to play myself into contention with nine holes left, whereas I wouldn't have been that way beforehand.

"It wasn't making that putt to get some momentum or hitting wedges in there close, (it was) making those two, three, four birdies in a row.

"I really turned a little bit of a corner of seeing more, of scoring better."

World number 13 Thomas must now apply those lessons after months of frustration.

"It was very frustrating," Thomas said. “After a few months where you're not performing as well as you feel like you should... it's pretty easy to get pissed off and understand what's going wrong. I'm starting to see a little bit of a light at the end of the tunnel."

Thomas said he doesn't feel he's in a slump now but did a month ago and had events where he doubted he could win or thinking he couldn't win.

"It sucks. It's terrible,' Thomas said. "I've never felt so far and so close at the same time. That's a very hard thing to explain and it's also a very hard way to compete and win a golf tournament.

"That's how you get out of it, just playing your way out of it and hitting the shots when you want to, making those putts when you need to, and then your confidence builds back up and next thing you know, you don't even remember what you were thinking in those times."