They'll look to expand their lead with a sixth straight win over the reeling Cardinals on Wednesday night.
The Pirates (64-42) have won eight of 11 behind a starting staff that has posted a 1.95 ERA during that span. They've compiled a 1.77 ERA in winning six of their last seven at PNC Park, helping raise Pittsburgh's home record among the majors' best to 35-18.
"We want more," manager Clint Hurdle said.
Jeff Locke (9-3, 2.15 ERA) will look to maintain both the Pirates' momentum and his own as he has gone 8-2 with a 1.70 ERA since April 23. The left-hander has yielded two earned runs or fewer in eight straight starts.
Locke has pitched around the plate all season long with little ill effect. He has walked three or more batters in 11 of his 20 starts, but he has a 2.37 ERA in those outings. That's partly due to his ability to pitch with runners in scoring position, as batters are hitting just .122 off him in such situations.
Friday was nearly a microcosm of his season - Locke struggled with control, walking six batters and allowing 14 baserunners overall, yet still gave up only two runs with a season-high nine strikeouts in 6 2-3 innings of a 2-0 loss to the Miami Marlins.
"Kind of sloppy," Locke said. "I felt really good, but just command wasn't there."
Locke has surrendered just three home runs in his last 17 starts.
While Locke ranks fourth in the majors with 57 walks, Adam Wainwright (13-6, 2.51) has issued 1.0 walks per nine innings, the best mark in the majors. The right-hander, who also is third in the NL with 145 strikeouts, has an 8.1 strikeout-to-walk ratio.
Unlike Locke, Wainwright has struggled keeping the ball in the park of late. After surrendering just four home runs in his first 17 starts, he has given up three in his last five, including one in seven innings of a 4-1 loss at Atlanta on Friday.
"Sometimes you just get beat," Wainwright said. "I had some pretty good stuff tonight."
Wainwright will look to be a stopper for St. Louis, which has lost a season-worst six in a row. During that skid, the Cardinals (62-43) have managed six runs and batted .155 with one home run. Carlos Beltran is 4 for his last 32 (.125), Daniel Descalso is 0 for his last 16 with five strikeouts and Matt Carpenter is 2 for his last 24 (.083).
"It's just one of those points in the season -- you have your ups and downs, and hopefully it's the only down we have," pitcher Lance Lynn said.
That struggling offense will be further impacted without All-Star Yadier Molina. The catcher, third in the NL with a .330 batting average, is expected to be put on the disabled list for a strained right knee that has been bothering him since June.
Wainwright will make his first start against Pittsburgh since he gave up seven runs and a season-worst 11 hits to the Pirates on June 29, 2012. One of those 11 hits was a home run by recent call-up Alex Presley, who had a walkoff single in the 11th inning of Pittsburgh's 2-1 victory to open Tuesday's twinbill and a pinch-hit triple in the nightcap, which the Pirates won 6-0.
Pittsburgh's bullpen pitched six scoreless innings on Tuesday. Mark Melancon, with a 0.89 ERA, has allowed just one run and 14 baserunners in his last 20 innings. Jeanmar Gomez, who completed the sweep with two hitless innings, has posted a 1.04 ERA since transitioning back into the bullpen.