As the champs return, the Pirates can take a big step toward ensuring they won't see the Giants in another one-game playoff.
Pittsburgh will try to put some distance between itself and third-place San Francisco as this four-game series gets underway Thursday night.
The Pirates (71-47) still have St. Louis in their sights in the NL Central - they trail the Cardinals by five - but hosting a third straight winner-take-all game at PNC, where they've won 32 of 42, is the likelier scenario.
The Giants (65-55) have a better chance of returning to the postseason as a division winner, sitting two behind the Los Angeles Dodgers in the West. A poor showing in Pittsburgh might make that route all the more probable, as San Francisco trails the wild card-leading Pirates by seven games and second-place Chicago by three.
Pittsburgh swept three from the Giants at AT&T Park in June while missing Madison Bumgarner, whose first look at the Pirates since completing the 8-0 wild-card win will come Friday.
Brandon Crawford, whose grand slam broke that game open, brings a career-high 13-game hitting streak into the opener.
Jake Peavy (3-5, 4.18 ERA) wasn't around for the June series either, part of an 11-week stretch in which he was recovering from a strained back. The right-hander posted a 2.74 ERA in his first seven starts after returning but struggled Saturday, allowing 12 baserunners and five runs over 5 2-3 innings of a 12-6 win over Washington.
This will be Peavy's first start against the Pirates since April 2009 - two months before Andrew McCutchen made his major league debut.
McCutchen had two more hits and a walk Wednesday as the Pirates beat Arizona 4-1 to take two of three. The five-time All-Star boosted his OPS at PNC Park to 1.155 since May 20, second best in the majors at home over the past three months behind the New York Mets' Lucas Duda.
He's 20 for 41 with eight extra-base hits in his last nine regular-season home games against the Giants.
San Francisco came into its series in St. Louis off a four-game sweep of Washington in which it totaled 28 runs, then only scored six while dropping two of three to the Cardinals. Half of those came in Wednesday's finale, but all eight Giants hits were singles in a 4-3 loss that followed San Francisco announcing it will put Hunter Pence on the disabled list with a strained oblique.
"That oblique is a hard one to figure out," manager Bruce Bochy said. "This is a moderate strain so we hope two weeks, he'll be ready to go."
The good news is Nori Aoki is expected to come off the concussion DL, giving the Giants back a leadoff hitter who also happens to lead the majors with a .368 road batting average. Aoki, though, is hitting .209 in 13 games since returning from a fractured fibula.
He's 3 for 7 off Charlie Morton (7-4, 4.36), who tossed six scoreless innings in Saturday's road start before allowing the Mets to tie it with three runs - two earned - on a pair of homers in the seventh of a 5-3 win.
The right-hander has allowed nine homers in 88 2-3 innings, matching the total he surrendered last season in 157 1-3.
He's kept the ball in the park in his last seven starts spanning 44 2-3 against the Giants, posting a 2.22 ERA.