Giants beat Reds 6-4, win NLDS on Posey's grand slam CINCINNATI - Buster Posey insisted all along -- even after San Francisco lost the first two at home -- that his team was far from done, no matter how long the odds. Turns out, he was right. And the NL batting champion had a lot to do with this rally of Giant proportions. Posey hit the third grand slam in Giants' postseason history, and San Francisco completed its startling comeback with a third straight win, holding off the Cincinnati Reds 6-4 Thursday in the decisive Game 5 of their NL playoff. San Francisco became the first NL team to overcame a 2-0 deficit in the division series, which began in 1995. The Giants won the final three games, all at Great American Ball Park, and advanced to the NL championship series to face either St. Louis or Washington. Ahead 6-0, San Francisco saw the Reds rally throughout the late innings. Ryan Ludwick hit an RBI single in the ninth off Sergio Romo. Then with two runners on and one out, Jay Bruce battled Romo before a flyout on the 12th pitch. Scott Rolen struck out to end it and the Giants raised their arms, hugged and huddled by the side of the mound, bouncing in unison. The Giants became only the eighth team to win a five-game playoff series after falling behind 2-0. Major League Baseball's changed playoff format this season allowed them to become the first to take a best-of-five by winning the last three on the road. With one swing, Posey made it possible. Posey connected off Mat Latos for his second career grand slam, putting the Giants up 6-0 in the fifth and sparking a joyous scrum in the San Francisco dugout. The ball smacked off the front of the upper deck in left field, just above Latos' name on the video board. Will Clark, in the 1989 NLCS, and Chuck Hiller, in the 1962 World Series, hit the other Giants slams in the postseason. Matt Cain and the bullpen held on, with more help from Posey. The All-Star catcher threw out Jay Bruce at third base to cut short a sixth-inning rally that had made it 6-3. The NL West champion Giants also had a pair of diving catches that preserved the lead in the eighth. In Cincinnati, the home-field meltdown felt sickeningly familiar. The Reds haven't won a home playoff game in 17 years. After taking the first two on the West Coast, all they needed was one more at home, where they hadn't dropped three straight all season. Once Posey connected, the NL Central champion Reds were the ones facing an unprecedented comeback. They've never overcome a six-run deficit in the playoffs, according to STATS LLC. Couldn't do it this time, either. The Giants won the World Series in 2010, when they never trailed in any of their three postseason series. They beat the Braves 3-1 in the division series, knocked out the Phillies 4-2 for the NL title, then took four of five from Texas for their sixth championship and first since moving from New York to San Francisco in 1958. They really had to scramble this season to get another shot at it. The bullpen took a huge hit when closer Brian Wilson blew out his elbow, and that was just the start. All-Star game MVP Melky Cabrera got a 50-game suspension in August after a positive testosterone test, taking a .346 hitter out of their lineup. The Giants have decided not to bring him back, even though he's eligible to return for the NL championship series. Two-time Cy Young winner Tim Lincecum pitched so poorly -- 15 losses -- that he got relegated to the bullpen for the division series. And don't forget that Posey was coming off a broken leg that wiped out most of his 2011 season, making a great comeback of his own. They pulled it all together during the streak of three improbable wins in Cincinnati. The Reds will remember the first inning of the series, where everything changed. Ace Johnny Cueto pulled muscles in his right side and had to leave the game. He wound up getting dropped from the playoff roster because of the injury. Latos pulled them through that opening game, pitching in relief on short rest for a 5-2 win. Latos came to Cincinnati from the Padres at a high price -- pitcher Edinson Volquez and three former high draft picks -- and with a clear purpose in the offseason. He was expected to take them to the next level. The right-hander allowed three hits through the first four innings, then fell apart in the fifth. Brandon Crawford had an RBI triple and scored on rookie shortstop Zack Cozart's error. A four-pitch walk and a single loaded the bases for Posey. As soon as Posey connected, Reds catcher Ryan Hanigan stood and turned away, unable to watch the ball head for the seats. Cincinnati's 17-year history of playoff futility was about to go on. NOTES: Posey's other career grand slam came on July 7, 2010 at Milwaukee. ... The Reds are 3-3 in winner-take-all playoff games, including 1-2 in best-of-five series and 2-1 in best-of-seven series. .. Teams that lose the first two games in a best-of-five series are 7-59 all-time. ... Cain gave up six hits and three runs in 6 2-3 innings, the longest appearance by a Giants starter during the series.