WASHINGTON -- One out away from absorbing a shutout defeat Saturday, the Giants instead left the nation's capital one victory away from reaching the National League Championship Series.

Engaging the Washington Nationals in a standoff that was equal parts endurance test and fight for survival, the Giants captured Game 2 of the NL Division Series, 2-1, on Brandon Belt's tiebreaking home run leading off the 18th inning. San Francisco forged ahead in the best-of-five series, 2-0, and needs one more victory to advance to the next postseason round.

The Giants figure to sustain their momentum as the series moves to San Francisco for Game 3 on Monday afternoon (2 p.m. PT on MLB Network). Game 4, if necessary, is scheduled for Tuesday night at AT&T Park. San Francisco's ace left-hander, Madison Bumgarner, is scheduled to pitch Game 3 -- though Washington will counter with its own formidable starter, right-hander Doug Fister.

The Giants know they must avoid overconfidence. They faced a 2-0 deficit in the 2012 Division Series against Cincinnati, losing both games at home, before roaring back to win the next three games.

"We were on the flip side of it two years ago," catcher Buster Posey said. "Hopefully we, as a group, can take that as a learning experience and keep the pedal to the metal."

The Giants have done exactly that. The outcome extended their NL-record postseason winning streak to 10 games.

San Francisco mustered two hits in extra innings until Belt faced right-hander Tanner Roark, Washington's eighth reliever. Belt worked the count to 3-2 before driving a fastball into the second deck. His objective in that plate appearance was much more modest, given his 0-for-6 performance to that point.

"I just wanted to get on base for the guys behind me -- 'get 'em on, get 'em over and get 'em in,'" Belt said. "Fortunately, I put a good enough swing on it."

Belt described the sensation he felt as he rounded the bases as "a great feeling. It's something you dream about your entire life. I'm very fortunate I was able to experience it."

Coincidentally, Belt was the designated hitter for the University of Texas on May 30, 2009, when the Longhorns participated in an NCAA-record 25-inning game.

Yusmeiro Petit earned the decision for the Giants with six stalwart innings of relief. The right-hander permitted just one hit while walking three and striking out seven. Rookie right-hander Hunter Strickland earned the save by blanking Washington in its half of the 18th.

Petit's experience in San Francisco's rotation enabled him to record what was essentially a quality start.

"I was trying to get as much as I could out of him," Giants manager Bruce Bochy said.

Said Belt, "He was just awesome. ... You can't say enough about what he did. He kept us in that ballgame."

The effectiveness of both bullpens helped make this the longest postseason game in franchise history, eclipsing San Francisco's 3-2, 13-inning loss to the New York Mets in Game 3 of the 2000 Division Series.

Moreover, at six hours, 23 minutes in length, this was the longest game in Major League postseason history in terms of time. For innings played, it matched the 18-inning contest in Game 4 of the 2005 NL Division Series, which resulted in Houston defeating Atlanta, 7-6.

"Epic battle," Bochy said.

Jordan Zimmermann's dominance was the story for much of the evening. The Nationals right-hander followed up his no-hitter last Sunday with another powerful effort, a three-hitter over 8 2/3 innings in which he retired 20 consecutive batters from the third through ninth innings. The Giants were so overmatched that they hit only one ball out of the infield during that stretch. They also advanced one runner as far as second base through the first eight innings.

As usual, the Giants refused to yield passively. One out from securing a complete-game, 1-0 triumph, Zimmermann walked Joe Panik on five pitches. Nationals closer Drew Storen relieved Zimmermann and surrendered Posey's first-pitch single. Pablo Sandoval sliced a double to left field that scored Panik. Posey also hustled home, but was tagged out as relays by left fielder Bryce Harper and shortstop Ian Desmond caught up with him.

Bochy requested a replay review, creating an unprecedented sense of drama given the two-out, ninth-inning, game-on-the-line situation. The review process played out efficiently, and the out call could not be overturned. Posey considered the review inconclusive.

"It's one of those where I think if I'm called safe on the field, it probably stands," he said.

Giants starter Tim Hudson excelled in 7 1/3 innings, allowing one run and seven hits while walking none and striking out eight.