Jordan Spieth feels he still needs to hit more fairways to have a realistic chance of winning the US PGA Championship, but the three-time major winner is understandably encouraged by his scoring after moving into contention on Friday.

Spieth has endured a disappointing slump in form over the last 12 months, failing to record a single top-10 finish since he placed ninth at last year's Open Championship having co-led with 18 holes to play.

However, a four-under 66 at Bethpage Black lifted the Texan to five under in the second major of 2019 and enhanced his hopes of completing the career Grand Slam at the third attempt.

Spieth finished his round as the closest challenger to leader Brooks Koepka, who duly birdied each of his first two holes in round two to move four clear at nine under.

"I still need to hit more fairways," said Spieth in a news conference. "I made a lot of putts, and I can't necessarily rely on that, and I scrambled really, really well. I think I was four for four out of the bunker and a couple others out of the rough.

"The ball just needs to find the fairway as often as it was [doing] for the guys around me; DJ [Dustin Johnson], Brooks. It's not going to be [going] as far as theirs, so I'd better be in as many fairways.

"I'm 100 per cent not hitting it as well as I did a couple years ago, but I'm hitting it a lot better than I did the end of last year, beginning of this year."

Asked to rank his putting, Spieth added: "I'm probably 90 per cent back to when I was at my best and the only difference maker is just speed control. I feel as good or better [from] 15 feet and in. I feel like I'm where I should be. I've put a lot of thought and work into it and the putting feels good."

Spieth does not believe the prospect of becoming only the sixth man to win all four of golf's modern-day majors will prove a distraction should he remain near the top of the leaderboard.

"It certainly hasn't [crept into my mind]," he explained. "I can't imagine it will because I haven't been in contention on a Sunday since The Open last year, and if I'm able to put some good work in tomorrow [Saturday]...then I will be in contention on Sunday. And at that point, it will be just more of trying to win a golf tournament.

"It won't matter to me what tournament it is. I'll be pleased to be in contention, knowing that the work I put in from being pretty far off has really come back nicely on a very difficult golf course. I imagine that will take pretty much most of my thought."