(Reuters) - Buster Posey powered the San Francisco Giants to a 6-4 win and an unlikely series comeback over the Cincinnati Reds on Thursday to book a spot in the National League Championship Series for the second time in three seasons.

A scoreless game was blown open when Posey capped a six-run fifth inning for the Giants with a grand slam that ended Reds starting pitcher Mat Latos's outing and silenced the home crowd.

After losing two games at home by a combined 14-2 score, San Francisco roared back with three consecutive road wins, making them the first National League team to win a best-of-five division series after losing the opening two games.

The Giants now advance to the penultimate stage of Major League Baseball's postseason for the first time since they won the World Series in 2010.

They will play either the St. Louis Cardinals or Washington Nationals in a best-of-seven series, starting on Sunday. (Reporting by Frank Pingue in Toronto; Editing by Julian Linden)