SAN FRANCISCO -- It took them two days to do it, but the San Francisco Giants finally have a win over the Cincinnati Reds this season.

Buster Posey hit a one-out home run in the bottom of the 17th inning, delivering the Giants a walk-off, 3-2 victory over the Reds in a game that started Friday night but ended in the wee hours Saturday morning.

The game was the second longest in AT&T Park history, trailing only an 18-inning marathon between the Giants and Arizona Diamondbacks in 2001. The one lasted 5 hours, 28 minutes.

"I think Buster'd had enough," Giants manager Bruce Bochy said of his star, who caught the entire 17 innings.

The win was the Giants' first in five tries this season against the Reds, who had dominated the first four matchups to the tune of 34-7.

Posey's homer, his sixth of the season, came off right-hander Robert Stephenson (0-2), the seventh Reds pitcher. It was Posey's third career walk-off homer, fourth walk-off hit and fifth walk-off RBI.

"I'm just glad it's over," Posey said. "It gets to a point where you're playing so many extra innings, your body hurts.

"We had some good swings. Balls hit to the wall. Fortunately, we were able to get one (over)."

Right-hander Cory Gearrin (1-1), the eighth Giants pitcher, got the win after pitching two scoreless innings during which he stranded five baserunners.

"You see you're the last guy," Gearrin said of the Giants' depleted bullpen. "You have to do what your teammates are doing. You don't want to be the guy who gives up the one."

Among the two hits Gearrin allowed in his two innings was the first of Stephenson's career, a bloop single to center field in the top of the 17th.

Until Posey's homer, neither team had scored since the fifth inning. The Reds didn't have any hits from the sixth through 11th innings; the Giants had only one from the 10th through the 15th.

Both bullpens pitched brilliantly.

The Reds used six pitchers in relief of starter Scott Feldman. They combined to allow just five hits, including the game-losing homer, in 9 1/3 innings.

"It just stinks. I feel like I let down my team," Stephenson insisted. "They pitched real well, played hard the entire game. For it to come down to one pitch, it's tough."

The Giants followed Johnny Cueto with seven pitchers who shut out the Reds on six hits for nine innings.

For the second night in a row, both starting pitchers turned the game over to their respective bullpens with the score tied 2-2.

Feldman left for a pinch hitter in the top of the eighth inning, having allowed two runs and seven hits in seven innings. He walked one and struck out two.

He had shut out the Giants on four hits Sunday in a 4-0 win in Cincinnati, besting Cueto in the process.

Cueto completed eight innings, giving up just five hits and two runs. He walked three and struck out six.

The Reds' two runs off Cueto came in the second inning after Denard Span had given the Giants a 1-0 lead in the first with a solo home run.

Scooter Gennett tripled in Scott Schebler, who had singled, to get Cincinnati even, then scored the inning's second run on Tucker Barnhart's sacrifice fly.

Following up his four-hit game in Thursday's series opener, Span had three hits, including the first-inning homer. Posey had two hits and Justin Ruggiano added two singles for the Giants, who totaled 12 hits.

Schebler had three of the Reds' 11 hits. Gennett and Joey Votto collected two apiece.

"To play 17 and lose, it's a lot different feeling in the Giants' clubhouse right now than it is in ours," Reds manager Bryan Price assured. "Both teams had an awful lot of opportunities to win that game, and no one could come up with the big hit until Posey did there."

NOTES: C Buster Posey's walk-off homer in the 17th inning was the latest in the Giants' West Coast history. ... Reds 2B Scooter Gennett's triple came in his next at-bat after he had tripled in the eighth inning of Thursday's win. The consecutive triples were a first for the Reds since OF Hernan Iribarren did it last September as a pinch hitter on back-to-back days against the New York Mets. ... Reds CF Billy Hamilton went 0-for-7, ending his eight-game hitting streak. He also had scored at least one run in 11 straight games. ... The leadoff home run was the 12th of CF Denard Span's career and the first by a Giant this season. ... Home plate umpire Tony Randazzo had to leave the game after 13 innings. He took a beating in the game, getting drilled in the right arm and stomach by foul balls, and in the right shoulder and chin area of the face mask with direct hits on pitches that appeared to have crossed up Posey. Clint Fagan moved in from second base to finish the game.