Rory McIlroy shot a two-under 68, which leaves him at six-under for the tournament and three shots behind the co-leaders.
He made only three pars over his last 11 holes and wound up with his best chance yet to win in America.
Johnson hasn't won since he captured his second major at St. Andrews, and he's just now starting to find some form with his short game. But on a Firestone course on which he has never seriously contended, he was efficient as ever.
Johnson and Pieters are tied at the top at 9-under 201.
After finishing fifth in the Tour Championship last September, Matsuyama won four of his next six events worldwide - including the WGC-HSBC Champions - and was second in the other two, before making a successful title defence in the Waste Management Phoenix Open in February. When that tournament never came together, organizers agreed to take the victor of the Order of Merit from the previous year. More importantly, he's giving himself chances and converted.
The Cavan girl, the current number one amateur in the world, carded a one-under-par 71 yesterday, which included three birdies and two bogeys.
"Just channeling my inner Jordan Spieth", he said.
Frustration proved the order of the back nine as McIlroy had to settle for mid-range birdie putts, many of which shaved the hole and stayed out.
" I hit a lot of good drives out there and it was humbling, but it always is with Rory and I'm used to it".
"I did leave a few out there", McIlroy said. "I think if I would have walked off today with a 65 or a 66, I would have felt that was a fair reflection of how I played. But I do miss the heat of things, and that's the goal is to get back in the heat by Saturday, Sunday next week".
"I know what I need to work on", Spieth said. Diamond is also due to carry the bag for his best friend at the fourth major of the year, the PGA Championship, at Quail Hollow next week.
Spieth, showing no signs of a let-down following his British Open triumph two weeks ago at Royal Birkdale, was two shots off the pace along with four-times major victor McIlroy, playing alongside new caddie Harry Diamond.