Buster Posey began a string of three straight two-out doubles in the sixth inning and Madison Bumgarner extended his streak of unbeaten starts to 10 Tuesday night, as the host San Francisco Giants opened a two-game interleague series against the rival Oakland Athletics with a tense, 3-2 victory. Scooter Gennett produced an insurance run with a sacrifice fly, while Will Smith survived a shaky ninth inning, helping the Giants reach .500 at 60-60 with a fourth win in their last five games. Bumgarner (8-7) and A's left-hander Brett Anderson (10-8) locked horns in a scoreless pitchers' duel until Oakland's Stephen Piscotty belted a two-out, two-strike homer off the San Francisco ace for a 1-0 lead in the fifth inning.