The Pirates scored twice in the first inning on a home run by Pedro Alvarez and could not manage another run until they got to Gonzalez Germen (0-1), the Mets sixth pitcher who was making his major league debut.
Mercer, however, sent a grounder up the middle to bring home Alvarez.
The win went to Vin Mazzaro (5-2), who struck out two in the top of the 11th. Pittsburgh has captured two in a row after losing four straight.
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