Pieters, who won a record four points on his debut at Hazeltine in 2016, holed out from a bunker on the ninth - his final hole - for one of his seven birdies to reach 12 under par, two clear of playing partner Ross Fisher.
Fisher carded a second consecutive 67 as the players took advantage of perfect conditions at Abu Dhabi Golf Club, with two-time winner Paul Casey a shot further back alongside Australia's Sam Brazel following a 65.
"I made a good eight-footer on four for par, which is probably the best putt I hit all day," Pieters told Sky Sports. "But the conditions are perfect, no wind. It's nice out there so you need to take advantage of the conditions.
"We have a decent game plan. I've done well here in the past [fourth in 2015 and second in 2016] and it suits me. We hit driver wherever we can and the rest of it is good ball-striking and going at a couple flags with wedges when you have them."
Pieters, Fisher and Casey were all members of the European team which won the EurAsia Cup in Kuala Lumpur last week, with Casey having rejoined the European Tour after a three-year absence in order to be eligible for September's Ryder Cup.
"I know this course very well," Casey said. "It seems like I've got good memories of every hole so it's about drawing from those past experiences.
"I suppose ultimately it means nothing in the grand scheme of things because you can't control what other guys do, but the biggest motivation is that (Martin) Kaymer has won here three times and I've got two. I see him in Arizona all the time and he doesn't let me forget it!"
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies