The right-hander will look to finish his season with a perfect record as the Giants host the San Diego Padres in a battle for third place in the NL West on Saturday.
The Giants (75-85), who moved into a tie for third in the division with their third straight win Friday, seek to avoid their third finish of fourth place or below since 1997.
The Padres, meanwhile, look to finish in the top three for only the sixth time since 1999.
Petit (4-0, 2.84 ERA) disappeared from the majors after going 3-10 with a 5.82 ERA for Arizona in 2009.
San Francisco picked him up in 2012, when he made one spot start for the Giants. He made an appearance out of the bullpen on July 23, but he didn't truly get his chance until Aug. 27, when he was inserted into the rotation.
He won his first three starts, capped by a one-hitter against the Diamondbacks, and San Francisco is 6-0 in his starts.
Petit has allowed one home run in 39 innings as a starter and is averaging 8.77 strikeouts per nine innings.
He was strong again Sunday, allowing a solo home run in 6 1-3 innings at Yankee Stadium, though he failed to record a decision in a 2-1 win.
The Giants have followed Petit's lead, finishing strong with 12 wins in their last 18 games.
Ryan Vogelsong was the latest member of a Giants staff that has an ERA of 3.26 over that stretch to excel, yielding two runs in six innings as San Francisco defeated San Diego 7-3 Friday.
The Giants rotation has allowed two runs or fewer in a season-best five straight games.
The Padres (75-85) entered Friday with the best ERA in the month of September, having surrendered two runs or fewer in 17 of 24 contests.
Eric Stults (10-13, 3.94) will look to continue his recent resurgence.
After the left-hander went 0-3 with a 7.16 ERA from Aug. 13 through Sept. 4, he is 2-0 with a 2.60 ERA over his last three starts.
On Monday, he held Arizona to a run in 6 1-3 innings of a 4-1 victory.
Stults now looks to avenge his last loss Sept. 4, when the Giants pounded him for seven runs - three home runs - in 4 2-3 innings of a 13-5 San Francisco victory.
Hunter Pence had one of those home runs and is 7 for 22 (.318) lifetime off Stults with seven RBIs, the most of any hitter off the left-hander.
He snapped an 0-for-27 skid with his career-best 26th home run Friday.
Pence, an impending free agent, appears to be close to staying in San Francisco. He and team president Larry Baer met in the team's clubhouse for more than 15 minutes after Friday's game discussing a potential new deal.
''They're about to get back to me so we're either really close or really far away,'' Pence said. ''I will know shortly. We'll just say we were talking.''
Manager Bruce Bochy, for one, would like to see Pence return.
''He's one of the more inspirational players that you can have on the club,'' Bochy said. ''He's full-throttle, the way he plays the game. He hit a home run on a big day for him.''
Stults is 1-2 with a 6.43 ERA in five starts this season against the Giants.