Cubs 4, Pirates 2: Alfonso Soriano belted a two-run homer in the eighth inning to snap a tie and help visiting Chicago complete its first road sweep since August 2010.
Josh Vitters and Anthony Recker also went deep for the Cubs, who registered their first series sweep since winning three straight at home against Arizona from July 13-15.
Andrew McCutchen hit a solo shot for Pittsburgh, which fell 2 1/2 games behind St. Louis for the second wild-card spot in the National League.
The Pirates tied the contest at 2-2 in the seventh, when reliever James Russell (7-1) entered with one out and runners on the corners and promptly committed a balk, bringing Brock Holt home from third. But Anthony Rizzo led off the eighth against Jason Grilli (1-6) with a single and Soriano deposited the reliever's first offering over the wall in center field for his 28th blast.
The homer gave Soriano 94 RBIs, his highest total since recording 95 in 2006 while with Washington. It also marked the second time in as many days Grilli failed to keep the game even, as he yielded a tiebreaking RBI single by David DeJesus in the eighth on Saturday.
Carlos Marmol loaded the bases in the ninth before nailing down his 18th save in 20 opportunities.
McCutchen opened the scoring in the first with Pittsburgh's lone home run on its 2-4 homestand. Vitters tied the contest in the fourth with a one-out solo blast, his first hit in 28 at-bats, and Recker launched his first career homer two batters later for a 2-1 Chicago advantage.
Holt collected three hits for the Pirates, who have lost seven of nine.
GAME NOTEBOOK: Cubs 2B Darwin Barney extended his errorless streak to 125 games, an NL single-season record for a second baseman. He eclipsed the previous all-time NL mark of 123 games set by former Cub Ryne Sandberg from June 21, 1989-May 17, 1990 on Saturday. The major-league single-season record for a second baseman is owned by Placido Polanco, who went 141 contests without an error in 2007 with Detroit. ... McCutchen has reached base safely in 48 of 52 career games against Chicago. ... Pittsburgh RHP Chad Qualls, who had been out since Aug. 25 with discomfort in his right big toe, was activated from the disabled list and retired both batters he faced in the eighth inning.