Buster Posey smashed a grand slam in a six-run fifth inning as San Francisco advanced to the second round of the Major League Baseball playoffs with a 6-4 win over Cincinnati on Thursday. The Giants became just the second National League team to win a first-round best-of-five series after dropping the first two games. Brandon Crawford collected two hits, an RBI and scored a run for the Giants, who won the World Series championship two years ago. The 1981 Los Angeles Dodgers also rallied from 0-2 to win their NL series against Houston. The NL West champions advance to face the winner of the Washington and St. Louis series in the other NL division series matchup for a chance to win the NL pennant. That series will be a best-of-seven. Pablo Sandoval finished with two hits and scored a run Thursday, while Angel Pagan also drove in a run for San Francisco. Giants\' starting pitcher Matt Cain lasted 5 2/3 innings, allowing six hits and three runs, while walking two and striking out five. Sergio Romo posted his first career playoff save by recording the last four outs of the contest.