Yusmeiro Petit outpitched Patrick Corbin last weekend in his first start against his former team.
Petit (2-0, 3.12 ERA) went 9-19 with a 5.05 ERA in 56 games - 35 starts - for Arizona from 2007-09 and did not pitch in the majors again until he made one start last year for the Giants.
Returning to Arizona for the first time since leaving the desert, Petit struck out a career-high 10 and gave up two runs over six-plus innings in an 8-2 victory Sunday.
Arizona manager Kirk Gibson said Petit "is a little bit more mature" than he was in his days with the Diamondbacks.
"Experience," Petit said with a teammate serving as translator. "Before I wasn't consistent with what I was doing. Now the experience is taking place in what I'm showing right now."
The right-hander was recalled from Triple-A Fresno to replace the injured Matt Cain on Aug. 23 and has posted 23 strikeouts over 17 1-3 innings this year while winning both of his starts.
"He's come up here and done all that we could ask for," Giants manager Bruce Bochy said.
Gibson gave Corbin (13-5, 2.96) two extra days of rest before Sunday, but the left-hander was charged with five runs over five innings. He was tagged for a career-high nine runs in 5 1-3 innings in his previous outing, a 9-5 defeat at Philadelphia on Aug. 25.
"I felt all right," said Corbin, who has never lost three straight starts. "I just wasn't really locating anything and left some pitches up."
Corbin is 0-1 with a 3.66 ERA in three 2013 starts against the Giants, and 0-1 with a 7.04 ERA in three in his career at San Francisco.
Gibson wasn't around for the end of the Diamondbacks' 4-2 victory Thursday in the series opener. He was ejected by plate umpire Tom Hallion for arguing a play in which the Giants' Pablo Sandoval scored on a sacrifice fly by Hector Sanchez in the fourth inning.
"I thought he missed it,'' Gibson said.
Paul Goldschmidt broke out of a slump with two hits and two RBIs as Arizona (71-68) remained seven games behind Cincinnati in the race for the NL's second wild card.
Goldschmidt had been hitless in 15 at-bats and gone six straight games without driving in a run before boosting his RBI total to 106 - the sixth-highest mark in franchise history. The slugger, however, is hitting .196 in the season series, with the Giants (62-78) winning 10 of 16.
San Francisco's Buster Posey missed his second straight game with a fracture in his swollen right ring finger suffered Tuesday in San Diego. It is unclear if he will return in this series.
``Throwing will be all right,'' Posey said. ``I haven't grabbed a bat yet, but I'll test that tomorrow. From what I'm hearing from the trainers and doctors, once it calms down I can play through it.''
Gibson plans to rest shortstop Willie Bloomquist, hitting .354, on Friday.