At Scottsdale, Arizona, Chris Marrero hit a three-run homer in the bottom of the ninth and drove in four runs as the Giants rallied after Madison Bumgarner gave up two runs on two hits in the top of the first.
Marrero, a 2006 first-round draft pick of the Nationals trying to revive his career as a non-roster invitee, had an RBI single in the fifth.
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Kevin Shackelford, the seventh Cincinnati pitcher, walked Orlando Calixte to open the ninth and Christian Arroyo blooped a single into left field before Marrero hit a line drive over the wall in left centerfield.
Patrick Kivlehan had a two-run single off Bumgarner in the first and Hunter Pence doubled in a run off Reds starter Rookie Davis. Davis, acquired from the Yankees in the Aroldis Chapman deal, is likely to open with Double-A Pensacola this year.