Before the 26,519 in attendance could settle into their seats, the Padres jumped all over Haren.
Back-to-back doubles to lead off the game gave San Diego a 1-0 lead, and after an RBI single by Adrian Gonzalez, a two-run homer by Kevin Kouzmanoff capped off the inning with the Padres on top, 4-0.
"Obviously, I didn't have good stuff today," Haren said.
Giving Peavy, who won last year's National League Cy Young Award, an early lead like that is more often than not a winning recipe for the Padres.
"That's why I was so down after the first inning," said Haren, who came into the dugout and told fellow pitcher Brandon Webb that he should have a batting practice screen out there because of how hard the balls were being hit. "Chances are you're not going to get too many runs off [Peavy], and I was down."
But neither he nor the D-backs were out, because Peavy was not on top of his game.
After he retired the first batter in the bottom half of the first, Peavy allowed a double to Augie Ojeda and walked Conor Jackson. Adam Dunn then worked the count to 3-2 before hitting a laser over the wall in right for a three-run homer.
Dunn, acquired from the Reds on Aug. 11, has given new life to the D-backs, and that was apparent with the lift his homer gave them.
"That's why we got Dunn right there," third baseman Mark Reynolds said. "That was a big home run right there."
Dunn is known for a patient approach at the plate, and that's something the D-backs have shown since his arrival.
"It just seems like it takes a lot of pressure off the guys throughout the lineup," manager Bob Melvin said. "Sometimes things like that can be contagious."
While neither pitcher had the kind of first inning they hoped for, Haren found a way to settle in.
"I was able to hold the lead even though it wasn't pretty," Haren said.
The right-hander kept the Padres off the board for the next four innings before allowing a run in the sixth, his final inning of work. Haren finished the night having allowed 11 hits over six innings. Not one of his better outings, but one that could have been a lot worse given the way it began.
"I think Danny held in there pretty well for us," Melvin said. "He's a fighter, and guys like that are able to get you deep into games and put those things behind them."
First innings have been a bit of a problem spot for Haren, and he and Melvin said it was the result of hitters being aggressive against him because they know he's going to throw strikes early and often.
"I face them again in five days, and hopefully I'll be able to make the adjustment and go out there and have a better game plan against them. ... They obviously came out with a plan to be very aggressive, and it worked out for them," Haren said.
Peavy (9-9) still held a one-run lead after the first, but the second inning proved to be no better for him as he allowed three more runs to give Arizona the 6-4 lead.
In the inning, Chris Snyder tied the game with a one-out homer and Jackson capped it with a two-out, two-run bloop single to right.