Five strokes off the pace after the second round at a sun-baked Whistling Straits, the Masters and U.S. Open champion fired a bogey-free seven-under-par 65 to vault within two strokes of Australian leader Jason Day.
Spieth stormed home with six birdies in the final eight holes to move into second place at 13-under 203 and earn a spot in the final pairing with Day on Sunday.
"On the back nine it was nice to get in the zone," world number two Spieth told reporters after negotiating the back nine in 30 strokes.
"The holes that I didn't birdie on the back nine, 10 I just barely missed, 14 I was short in the heart.
"And then 15 was a great two-putt. So, yeah, I'm very, very pleased to have a chance to win another major."
Spieth is seeking to join Ben Hogan and Tiger Woods as the only players in the modern era to have won three majors in a single year.
The 22-year-old Texan was frustrated after recording just one birdie in his first 10 holes as he slipped down the leaderboard on a day of low scoring.
"I was impatient on the front nine," he said. "I felt like I was playing some solid golf ... I just wasn't scoring.
"My score did not reflect the way that I was playing, which has been a little bit of the story this week, minus two stretches of nine holes.
"All in all, it's been really two fantastic nine holes in my mind. And tomorrow, if I can limit the mistakes and hit the fairways like we did it today, we'll be in really good shape."
A Spieth victory would emulate Hogan, who was the first to complete a hat-trick of majors in one year, in 1953, and Woods, who did so in 2000.
"Tomorrow it will be drawing on all the positives that we've done, when we did win, why did we win, what was there, and how are we going to make it happen again," said Spieth.
(Editing by Andrew Both)
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