Yoervis Medina took the mound and after Gaby Sanchez grounded out, Todd Snider was issued an intentional walk to get to Mercer. But the Pittsburgh shortstop foiled the strategy by lining a 1-1 pitch up the middle for a run-scoring hit.
Snider went to third on the play and scored an insurance run when Medina uncorked a wild pitch with Starling Marte at the plate.
With their sixth straight win, the Bucs pulled to within a half-game of National League East-leading Cincinnati.
Vin Mazzaro (4-2) earned the victory and Mark Melancon picked up his second save despite allowing a pair of hits in the ninth.
Raul Ibanez clouted his 18th homer of the season for the Mariners.