Mercer's drive off the Clemente Wall in right field scored McCutchen, who had led off with a double off former Pittsburgh closer Grilli, who next issued an intentional walk to Starling Marte and fanned Pedro Alvarez.
Lefty Francisco Liriano went seven innings for the Pirates, rationing six hits, but squandering a 2-1 lead on a solo homer in the sixth by Juan Uribe.
"Every once in a while, with runners on, Frankie gets quick with his delivery and loses that release point," said Pirates manager Clint Hurdle, mindful that Liriano forced in Atlanta's first run with a bases-loaded walk in the second. "He recalibrated after that and was in sync, up until Uribe's swing."
Atlanta starter Williams Perez still had a 1-0 lead when he departed in the fifth two batters after Josh Harrison hit a sharp grounder off the inside of his left ankle. Alvarez's two-run single later in the inning off Luis Avilan gave the Bucs a 2-1 lead.
Oh, Mercer: Mercer's double in the 10th inning not only capped off the win, but put a bow on his big day at the plate as he went 3-for-5 with two doubles overall. Mercer has now hit safely in his past three games, going 6-for-15 (.400) in that span. More >
Searage sets Liriano straight: Liriano was in an early mess. He'd walked Jonny Gomes to load the bases with one out in the second, then Ryan Lavarnway to force in a run. He'd thrown eight straight balls. Pitching coach Ray Searage came out for a visit, and one pop up and one strikeout later, the Pirates were back in their dugout. More >
Perez's delayed exit: Perez had completed four scoreless innings before his left ankle was squarely struck by Harrison's comebacker in the fifth inning. The right-hander remained on the ground for a few minutes in pain and then surprisingly remained in the game. But he exited after issuing a walk, being charged with a balk and hitting McCutchen with a pitch. The two runners he left on base scored on Alvarez's two-out, two-run single off Avilan.
"It did affect me after I got hit," Perez said. "I was a little scared and a little timid to get on that foot. That caused me to be a little more erratic." More >
Polanco's miscue: The Pirates would have had a bases loaded threat in the fourth -- had Gregory Polanco not run to third base when it was already occupied by a teammate. With Mercer on second and Polanco on first, Chris Stewart singled to deep short to send Mercer to third -- and Polanco barreling around second also to third. After he reversed field and a brief rundown, Polanco was tagged for the third out.
"First time I've seen a guy do a pop-up slide at third with one of our runners on third," said Hurdle, able to consider the lighter side of a potentially costly play with the win in the books.
The victory kept the Pirates perfect at home against the National League East. The Bucs are 10-0 versus NL East clubs in PNC Park.
The Pirates' successful challenge of an out call at first base in the fifth inning immediately preceded the two-run single that gave them a 2-1 lead. With two on and two outs, Marte hit a roller to second and was called out on Jace Peterson's toss to first. Clint Hurdle's challenge of first-base umpire John Tumpane's call led to a reversal -- replays clearly showed Marte crossing the bag before the ball was in Chris Johnson's glove -- and Alvarez followed with his hit up the middle.
Braves:Julio Teheran will take the mound when Atlanta and Pittsburgh resume their three-game series on Saturday at 4:05 p.m. ET. Teheran has completed at least seven innings in three of his past four starts and he limited the Mets to one hit over seven scoreless innings in his most recent start.
Pirates:Charlie Morton gets a chance to wipe out the memory of his ill-fated last start (nine runs in two-thirds of an inning in Washington) when he takes the hill again to face the Braves in the middle game of the series. Morton scored his first career win over Atlanta three weeks ago, albeit allowed four runs (three earned) in five innings of a 10-8 decision.