Tabata and Michael McKenry each finished with three RBI and Clint Barmes capped a five-run fifth with a two-run single for the Pirates, who notched a second victory in three games against the Mets.
Pittsburgh banged out 16 hits, including two each from Mercer, Starling Marte, Brandon Inge, McKenry and Barmes.
Francisco Liriano (1-0), making his Pirates debut after fracturing his right forearm in December, limited the Mets to one run on six hits over 5 1/3 innings. The lefty walked two and struck out nine.
New York starter Jonathon Niese (2-4) was lit up for eight runs on eight hits, lasting just 4 1/3 frames.