The Giants missed a chance to close ground on Colorado in the National League wild-card race, remaining three games behind the Rockies. San Francisco will host Colorado in a three-game series beginning Friday.
Petit (3-8) ended a three-start losing streak, striking out five and walking two.
Three relievers combined to give up one hit over the final three innings.
Brandon Allen drove in four runs, three of them with a fifth-inning homer that gave the Diamondbacks an 8-0 lead. He tacked on an RBI double in the ninth.
Rookie Joe Martinez (3-2) took the loss, surrendering eight runs on nine hits in 3 1/3 innings.
Martinez is 2-2 in five starts since coming back from being struck on the head by a line drive April 9 against Milwaukee.
Copyright 2009 by United Press International.
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