McIlroy, playing his first tournament since sacking longtime caddie J.P. Fitzgerald as he tries to turn around a so-far disappointing season, chipped in for birdie at the 16th en route to a one-under 69 while Japan's Matsuyama and America's Johnson both shot 67.
The 25-year-old will head to Quail Hollow in North Carolina for the final major of the year, the PGA Championship, one of the favorites after top-15 finishes in the previous three majors this year. This week and beyond, the consensus is growing that although others will no doubt intrude from time to time on their private party, it is these...
"I was three behind going into Sunday last time and I think the took the lead by the sixth tee box", said McIlroy.
" I put myself in good positions out there and had some great chances coming in the last six holes and didn't convert any of them".
"It's just a matter of doing it all over again tomorrow, seeing a few more putts drop and putting a bit of pressure on the leaders and we'll see what happens".
However, after finding rough off the ninth tee and coming up short of the green, the four-time major victor hit a clumsy pitch which ran through the green and failed to get up and down to save par.
Pieters, who is from Belgium and played college golf at the University of IL, made five early birdies to vault into the lead after eight holes, but he stalled from there and parred 11 holes the rest of the way.
First-round leader Pieters had threatened to run away from the chasing pack when he birdied three of the first four holes in an awesome display of ball-striking, and another pure iron from over 200 yards to eight feet at the eighth set up another gain.
"My ball striking was good, driver was not good. and I managed to score really well, so pretty happy", said Pieters, a three-time victor on the European tour.
Johnson will be seeking his first title since he captured his second career major at the British Open at St Andrews in 2015.
Thomas Pieters finds golf to be a lot more enjoyable when he's playing from the fairway.
But from 250 yards away the big-hitting Belgian fired his approach onto the green to set up a two-putt birdie, before nearly spinning his approach to the third back into the hole.
The two-time major victor outshone playing-partner McIlroy, particularly with the putter, although his round was not without its blemishes as he mixed eight birdies with three bogeys overall and carded only one par from the fifth through to the 11th.
Open champion Jordan Spieth shot even-par to stand another shot back, with Australian Jason Day among six players bunched at 137. Lexi Thompson, who started the third round two shots behind, stumbled to 74 and wound up 10 shots back.