That tandem hopes to keep swinging the bat well while McCutchen looks to turn things around Thursday night when the Pirates open a four-game road series with the Arizona Diamondbacks.
One day after taking a one-half game lead for the NL's second wild-card spot and climbing within one game of Central-leading Milwaukee, the Pirates (57-50) had their three-game win streak end in Wednesday's 7-5 loss at San Francisco.
Both members of Pittsburgh's red-hot left side of the infield hit two-run homers to help build a 5-4 lead, but a base-running gaffe in the sixth killed a potential big inning and turned the tide in the Giants' favor.
Harrison is 8 for 17 while homering in each of the last four contests - he hit just five through his first 87 games - and Mercer is 6 for 15 with three runs over that span after hitting his first homer since June 21. McCutchen, meanwhile, has gone 1 for 18 in his last five and hasn't gone deep in 13 straight but leads the team with 17 home runs.
Harrison has a chance to become the first Pirates player to homer in five straight since Jose Castillo in May 2006.
"We call it game-time pop," said manager Clint Hurdle, whose club has eight homers in the last four games.
Mercer has a good chance to extend his tear since he's 8 for 13 in his last four versus Arizona and homered in his only at-bat against Josh Collmenter in 2012.
Collmenter (8-5, 4.03 ERA) will have to regroup following arguably the worst performance of his career. He surrendered six runs and 11 hits over 2 2-3 innings Saturday in a 10-6, 10-inning win at Philadelphia.
The right-hander looks to get on track in the comforts of Chase Field, where he's 5-2 with a 2.89 ERA in 10 starts. He's 3-0 in his last four games there, yielding three runs in 20 innings, and Arizona is opening a 10-game homestand.
Collmenter has a 1.59 ERA in four career outings versus Pittsburgh.
Pittsburgh's Jeff Locke (2-2, 3.54) had a 2.38 ERA during an eight-start stretch before allowing a season high-tying six runs and a season-worst 10 hits, including three homers, in Saturday's 8-1 loss at Colorado.
The only time he's surrendered more runs came in the first of his two career starts against the Diamondbacks, allowing eight runs and 10 hits in 2 2-3 innings of a 15-5 loss Aug. 17, 2013. This year, however, he held them to two runs in eight innings before the Pirates rallied to win 3-2 on July 1.
Pittsburgh won two of three in that series despite Paul Goldschmidt going 7 for 11 with two homers and a triple. He's 4 for 6 lifetime off Locke.
The All-Star first baseman had been 1 for 16 with seven strikeouts in his prior four games before hitting his 19th homer in Wednesday's 5-4 win at Cincinnati.
Didi Gregorius added a key two-run homer in the ninth inning and went 6 for 14 in the series as Arizona (47-61) won two of three.
"That's something we haven't been doing a lot lately," said manager Kirk Gibson, whose club hadn't homered in its previous three contests while totaling four runs.
Gibson needs one win to surpass Bob Melvin for the most in franchise history with 338.