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 on 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.’’
‘‘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 NLCS will be Sunday, either in Washington against the Nationals or in San Francisco against the Cardinals. In the meantime, the Giants will stay in Cincinnati until their next opponent is determined Friday night.
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’s 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 had never overcome a six-run deficit in the playoffs, according to STATS LLC.
Couldn’t do it this time, either.