Eighth-ranked Jordan Spieth made progress on swing struggles in Friday's second round of the PGA Championship but admits he needs a "special" weekend to complete a Career Grand Slam.

The three-time major winner fired a bogey-free four-under par 66 to stand on three-under 137, seven off the pace after 36 holes at Bellerive Country Club in the year's final major tournament.

"I need something really special," said Spieth. "My expectations are lower than they are when I'm in form and I'm okay with that. It's just a process and I've got to be patient with it getting back."

Spieth, the 2015 Masters and US Open winner and last year's British Open champion, needs only the PGA to own each of golf's four major crowns, a life's feat achieved only by legends Tiger Woods, Jack Nicklaus, Gene Sarazen, Gary Player and Ben Hogan.

But the 25-year-old American hasn't looked like contending in the 100th PGA, starting Thursday with a double bogey on his way to a 71, in part because he struggles to trust his swing.

"Today was really a lot of progress," Spieth said. "I wasn't sure going into the first tee. I thought that anything could happen today... but it was a really solid round."

Spieth said in May he made changes before the Players Championship and spent two months doing something that was ultimately a step in the wrong direction.

"It cleared stuff up temporarily, but in fact pushed me further away from what I should have been doing," he said. "So I spent literally two months nailing in the wrong thing and I'm trying to climb back out of it.

"It's tough to trust on the golf course and today I had no choice. You have to trust it and started to clear up a bit of it. I'm just going to have to play, I just have to deal with it."

Since finishing third at the Masters, Spieth has missed three cuts and his only top-10 effort was a share of ninth at last month's British Open.

Spieth says the proper swing position still feels awkward to him.

"I'm just in that unfortunate position where I have to feel awkward when I'm over it before I even start the swing," Spieth said. "So through the swing it feels even more awkward.

"Given that, today was a really solid day. But I need better each day on the weekend to have a chance."