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CINCINNATI -- Not just any comeback would get San Francisco back to playing for a pennant. It would take one of Giant proportions.

And Buster Posey believed it could happen. Even after the Giants left the West Coast down two games, the National League batting champion insisted his team could pull it off, despite the long odds.

With one swing, he got everyone else believing it, too.

Posey hit the third grand slam in Giants' postseason history Thursday, and San Francisco pulled off an unprecedented revival, moving into the championship series with a 6-4 victory over the Cincinnati Reds.

"You don't want to be in a lose-and-you're-out scenario," reliever Jeremy Affeldt said, wearing a brace on his left wrist so he didn't hurt it in the champagne-flavored clubhouse celebration. "We've been in that situation for three days. We're probably going to sleep well tonight."

They'll play either Washington or St. Louis for the NL pennant, Sunday, not caring at all who they face.

"We could go up against anybody at any time," shortstop Brandon Crawford said. "Being down 2-0 and coming back and winning three at their place, it's an unbelievable feeling."

Game 1 of the NL championship series will be Sunday, either in Washington against the Nationals or in San Francisco vs. the Cardinals. In the meantime, the Giants will stay in Cincinnati until their next opponent is determined Friday night when the Cards and Nats play Game 5.

The Giants became the first NL team to overcome a 2-0 deficit in the division series, which began in 1995. 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.

Posey's second career grand slam off Mat Latos put the Giants up 6-0 in the fifth and sparked 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.

For the first time in the series, the Giants could exhale.

"I don't think anybody gave up," Posey said.

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 snuff out a sixth-inning rally that cut it to 6-3. The Giants had a pair of diving catches that preserved the lead in the eighth.

There was more drama in the ninth. Ryan Ludwick singled home a run off Sergio Romo. With two runners aboard, Romo fanned Scott Rolen to end it.

The Giants raised their arms, hugged and huddled by the side of the mound, bouncing in unison.

"It was a spectacular moment," outfielder Hunter Pence said.

In Cincinnati, the home-field meltdown had a sickeningly familiar feeling. 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.

"You get tired of the disappointments, but then you get over it," manager Dusty Baker said. "It hurts big-time."

Once Posey connected, the Reds were the ones facing a steep comeback. They've never overcome a six-run deficit in the playoffs, according to STATS LLC.

Couldn't do it this time, either.

"Buster Posey's swing was a series-changer," said Reds star Joey Votto, standing on second base when the game ended. "That made it very difficult to come back. You know they're going to throw the kitchen sink at us."

The Giants never trailed in any of their three postseason series when they won it all in 2010. 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 World Series title and their first since they moved 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 NLCS.

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.

"Unreal," Romo said, with champagne dripping off his scraggly beard. "That guy's definitely the MVP of our team. We believe he's the MVP of the league. We wouldn't be here without him, that's for dang sure. He's the one that's been the face of the team all season long. What a great story with all he's been through last year."

The Reds won't forget the first inning of the series, when 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. 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 he 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.

Game 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.