Left fielder Brandon Moss grabbed another glove from the dugout and moved in to make a five-man infield, then Tomlinson found a hole against Siegrist (5-1).
Marlon Byrd hit a grand slam off 15-game winner Michael Wacha in the third inning for the Giants, who kept pace 2 1-2 games behind NL West-leading Los Angeles.
Javier Lopez (1-0) got Moss to line out to second in the ninth and Tomlinson quickly fired to first to double up Stephen Piscotty, who was off the bag.
Byrd's eighth career grand slam gave the Giants eight for the season, a franchise record. Byrd last hit one on July 9, 2013, while with the Mets facing the Giants at AT&T Park.
