Zito did not get the decision as the win went to Sergio Romo (3-1), who notched the final out in the sixth inning.
Eugenio Velez drove in two of the Giants' runs. Pablo Sandoval had the other San Francisco RBI.
Charlie Morton (2-3) took the loss despite a solid six-inning start in which he gave up two runs on six hits and fanned five.
Morton was staked to a 1-0 lead in the top of the first when Delwyn Young singled to drive in Andrew McCutcheon, who had led off the game with a double.
Velez tied the score with a solo home run in the second and then put San Francisco ahead 2-1 in the sixth with an RBI double that scored Fred Lewis.
Sandoval drove in the Giants' third run with a single that scored Andres Torres from third.
Pittsburgh added a run in the eighth when Andy LaRoche scored on a sacrifice fly by Luis Cruz.
Copyright 2009 by United Press International.