CINCINNATI — The entire season was a comeback for Buster Posey, so he didn’t think anything of it when San Francisco needed one of the biggest yet to play for a pennant.

He led them to one of Giant proportions.

The National League batting champion hit the third grand slam in Giants postseason history on Thursday, sending San Francisco back to the championship series with a 6-4 victory over the Cincinnati Reds.

They will play Game 1 on Sunday, either in Washington against the Nationals or in San Francisco against the wild card St. Louis Cardinals. They planned to stick around town until the Nationals-Cardinals series, tied 2-all, is decided on Friday.

Matter which one?

“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.”

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.

San Francisco won the World Series in 2010 without trailing in any of its postseason series. The Giants took four of five from Texas for their sixth title and their first since they moved from New York to San Francisco in 1958.

They’ve really had to scramble this season to get another shot at it.

Their 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.

“Unreal,” said Sergio Romo, who fanned Scott Rolen with two runners aboard to end it. “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.”

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 before Romo got Rolen swinging 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.

Nationals 2, Cards 1

WASHINGTON — Hours before Jayson Werth’s game-ending homer on a 13-pitch at-bat extended the Washington Nationals’ surprising season, before a bullpen trio got eight consecutive outs via strikeouts, before Ian Desmond’s acrobatic catch, a guy who isn’t even on the playoff roster fired up a team facing elimination against the defending champs.

Mark DeRosa, a spare-part utility player who made his major league debut 14 years ago, grabbed the microphone of his clubhouse karaoke machine and quoted from Roosevelt’s rousing 1910 “Man in the Arena” speech — aiming to make the Nationals think they could beat the St. Louis Cardinals and set up a Game 5 in their NL division series Friday night.

“Our backs were against the wall. I wanted to say something that brought us together, a band of brothers who go out fighting and see what happens,” DeRosa explained.

Worked wonders, apparently.

“Epic,” was closer Drew Storen’s description. “It’s the stuff movies are made of.”

So was the scene in the bottom of the ninth inning in Game 4, when Washington beat St. Louis 2-1 to tie their NLDS.

Joyous, bouncing teammates waiting to greet him, the red-clad crowd raucous as can be, Werth yanked off his batting helmet with two hands and thrust it a dozen or more feet overhead before leaping onto home plate after his big hit.

A little less than two years ago, the Nationals showered Werth with millions, persuading him to come show them how to win. With one swing of his black bat at dusk Thursday, Werth wiped away whatever disappointments marred his early days in D.C.

Werth led off the bottom of the ninth inning against reliever Lance Lynn by fouling off pitch after pitch before sending the ball beyond the wall in left field, giving the Nationals a tense victory.

“That’s the way that game should have ended: Jayson Werth hitting a home run,” Nationals manager Davey Johnson said. “He has not hit that many this year. ... Unbelievable. Great effort on his part.”

The best-of-five series will end Friday night in Washington, with the winner advancing to face the San Francisco Giants in the NL championship series. The starters are a rematch of Game 1, which Washington won 3-2: Gio Gonzalez on the mound for the NL East champion Nationals, and Adam Wainwright for the wild-card Cardinals.

“It’s what you play all season for, and what you work out all winter for, and what you get to spring training early for,” Werth said. “We have a chance tomorrow to take that next step. I know my teammates will be ready. And the city will, too.”

The homer was Werth’s first of the series, the 14th of his postseason career. He won the 2008 World Series and a string of division titles with the Philadelphia Phillies, then moved to Washington before last season as a free agent on a $126 million, seven-year contract that stunned much of baseball.

He managed to hit only five homers with 31 RBIs in 2012, missing 75 games because of a broken left wrist. Last year, his first in Washington, Werth hit .232 with 58 RBIs, and there was grumbling about his worth.

That vanished when Werth circled the bases, raising his right index finger in a “No. 1” gesture, while the announced attendance of 44,392 roared, and other Nationals raced out of their dugout to greet him.

“I’m just happy that these fans got to see it, because obviously he had a rough year last year, and he got hurt this year, and I don’t think the fans realize how good of a player Jason is,” Nationals third baseman Ryan Zimmerman said. “For him to have a moment like this in front of the home fans, and in front of this atmosphere, I couldn’t be happier for him. He deserves it.”

Werth’s arrival certainly coincided with a quick turnaround: The Nationals lost 100 games in 2008 and 2009, but led the majors with 98 wins this year.

“I knew that a winning ballclub would bring the fans,” Werth said, “and here we are, two years later, and they’re showing up and it’s awesome.”

According to DeRosa, Werth heard the pregame oration and came running to hear.

“I feel like if they’re going to keep me around and keep me here, there’s a reason for it,” DeRosa said. “I’ve been in these games. I’ve grinded.”

Roosevelt’s speech, which he gave in Paris, includes the lines: “The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood ... and who at the worst, if he fails, at least fails while daring greatly, so that his place shall never be with those cold and timid souls who neither know victory nor defeat.”

Werth’s shot provided a sudden end to a classic postseason contest filled with tremendous pitching. Each team managed only three hits. No Washington batter ever came to the plate with a runner in scoring position. Both of the host’s runs came on solo shots, including Adam LaRoche’s in the second off Kyle Lohse.

Lynn, usually a starter for St. Louis but a reliever in these playoffs, was making his third appearance of this series.

“Heater. He beat me,” Lynn said. “I’ve had success this series with him, and, you know, everyone in the stadium knew what I was throwing there.”

Especially Werth.

“It was just a matter of time,” Lynn added. “I was challenging him, and he was up for it.”

The righty was the Cardinals’ third pitcher — facing only one batter — and manager Mike Matheny was asked afterward why he didn’t use closer Jason Motte.

“If we were at home, it would have been a very easy decision to bring in Motte,” Matheny said, explaining that if he used up his closer and St. Louis went ahead later in the game, a reliever not used to getting a save would have needed to try.

“Had a lot of confidence in Lance. He came in throwing the ball well,” Matheny added. “Werth just put together a very good at-bat.”

Cardinals batters decidedly did not down the stretch. Washington’s Jordan Zimmermann — the Game 2 starter making his first relief appearance in the majors — Tyler Clippard and Storen combined for eight Ks in three innings. Storen pitched the top of the ninth and got the win.

“All of them were throwing harder than I’ve seen them throw,” Johnson said.

Storen walked No. 8 hitter Pete Kozma with two outs, before getting pinch hitter Matt Carpenter on a twisting, stumbling overhead catch by shortstop Ian Desmond, who wound up sliding face-down along the grass in short left field. When Desmond rose, he chucked the ball high into the stands and yelled.

Moments later, Werth had all the towel-twirling spectators yelling, too, thanks to the way he turned on a 96 mph fastball. For much of the game, the hometown fans were rather quiet, perhaps dreading a sooner-than-expected end to their team’s better-than-expected year.