It turned out to be a smart move.
Duke threw six scoreless innings before Mark Reynolds hit a monster two-run homer some 476 feet onto the patio in front of the Friday Front Row Sport's Grill in left field, the seventh-longest home run at Chase Field.
"He crushed it," Duke said. "That's all there is to it."
Duke (2-0) struck out only two and walked two but baffled Arizona's batters into groundouts and popouts.
"He got balls on the ground, was consistent, changed speed," Pirates manager John Russell said. "That's what Zach's capable of doing, and he pitched a really good game."
Counting the last four innings of his opening day victory over the Los Angeles Dodgers, the left-hander threw 10 straight scoreless innings before Reynolds emphatically ended the streak.
The Diamondbacks love to hit fastballs, and Duke kept them off balance with a steady diet of off-speed pitches.
"We were able to use their aggressiveness against them," he said.
Haren (1-1) was off his game from the start, giving up five runs in the first four innings, capped by Iwamura's two-run homer in the fourth.
"I left a lot of balls over the middle of the plate, too many pitches up in the zone," Haren said. "That's not a recipe for success. I made some adjustments as the game went along but I dug myself too big a hole to get out of it."
The Pirates hit the ball hard for the second game in a row, only this time they weren't right at defenders.
"Of all the big league parks I've played in, just seeing the ball this is by far the best," Clement said. "The batter's eye is just huge out there."
Octavio Dotel pitched a scoreless ninth for his first save in his first opportunity.
The Diamondbacks had the bases loaded and no outs in the eighth but came away with just one run on Tony Abreu's sacrifice fly. Brendan Donnelly struck out Justin Upton, and left-hander Javier Lopez fanned Adam LaRoche to end the inning.
Arizona catcher Miguel Montero left in the second inning after injuring his right knee running out a grounder to the pitcher. He was trying to avoid Duke's tag when he landed awkwardly just before reaching the bag.
Diamondbacks manager A.J. Hinch said the knee didn't have a lot of swelling or pain, but Montero said it was uncomfortable. An MRI was planned.
"Anytime you talk about knees, it's scary," Hinch said, "but we'll hope for the best and act accordingly."
Before Reynolds' big home run, the only hits off Duke were singles by Abreu in the first and Reynolds in the fifth.
Haren, who gave up a run and three hits through seven innings in his opening day win against San Diego, allowed six runs -- five earned -- and nine hits in 6 2/3 innings. He had nine strikeouts and two walks, throwing 108 pitches. He fanned Ryan Doumit four times.
Arizona RHP Brandon Webb is scheduled to play catch Monday for the first time since receiving a cortisone injection on March 30. Webb has had a slow recovery from shoulder surgery. ... The Pirates improved to 3-8 at Chase Field over the last four years. ... Abreu started his first game for the Diamondbacks. ... Duke, who also started 2-0 last season, won consecutive games for the first time since last May 17. ... Counting last season, Haren has allowed five earned runs in five of his last six starts.