England's Paul Casey finished bogey, bogey, double bogey to drop back to level par but remains the highest placed British player.
Kevin Kisner bogeys the last but clings to a one-stroke lead on seven under par after posting a one-over-par 72. American Chris Stroud is best placed among those yet to complete the round, five-under with five holes left.
It is probable that Day, now tied for 16th, cost himself a shot in what could be a wild, wide-open finish Sunday in the final round at Quail Hollow Club.
The 33-year-old Kisner attacked the par-five holes in second-round action, carding an eagle on the seventh and birdies on the 10th and 15th.
At one point Friday, Fowler went over to talk to National Basketball Association star Andre Iguodola - who was following the group that included both Fowler and Rory McIlroy - while waiting to hit a shot.
"I was trying to do just that, trying to play aggressive, see how many birdies we could make", Spieth said. "He's working hard. And I feel like he's the hardest worker out here right now, just because he wants to win", said Day.
Halfway through this tournament, Fowler knows he has another chance to get his first major.
He could have got away with a bogey or worse a double but he went in for a shot that he expected to miss the tree and the bushes within hand-shaking distance.
He was forced to chip out and after eventually reaching the green, three-putted for an eight.
Matsuyama, who shot 73, has the chance to become the first Japanese golfer ever to win a men's major.
With 18 holes left and the way players finished on Saturday, I'm convinced anything could happen.
Neither had threatened the top of the leaderboard in the opening rounds but day three was where their challenges ended.
After 17 holes, Finau was 1-under par but made bogey on the final 18th.
"Kis is on fire right now", said McIlroy.
"The greens were a lot slower than what I expected, but they were firmer as they had dried up".
The Canadian, who has never won a professional tournament on American soil, played holes No. 13-16 in a ridiculous 6 under par, including back-to-back eagles on 14 and 15.
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