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.
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.