Daniel Mengden pitched into the eighth inning for his first major league win, backed by Marcus Semien's three-run homer and four RBIs, and the Oakland Athletics beat the San Francisco Giants 8-3 on Monday night to open the Bay Bridge Series. Semien connected in the decisive second and Jed Lowrie and Khris Davis added RBI doubles against Jeff Samardzija (8-5) in that five-run inning, then Semien added a run-scoring double in the sixth. The right-hander had pitched well enough to win during his initial three major league starts but received only four runs of support combined.