SAN FRANCISCO -- Brandon Belt hasn't done much as a starter this season. But off the bench, he was a hero Monday night.
Buster Posey tied the game with a two-run home run in the eighth inning and Belt won it with a one-out RBI single in the ninth as the San Francisco Giants outlasted the Arizona Diamondbacks 5-4 in the opener of a three-game series between the second- and third-place teams in the National League West.
Belt, who entered the game in the top of the ninth as part of a double-switch, sliced the game-winner over shortstop against Arizona's fourth pitcher, Tony Sipp (1-1), scoring Andres Torres from second base and strengthening the hold on second place for the Giants (13-7).
"We're never out of the game," assured Belt, who didn't start for the second game in a row and watched the Diamondbacks use home runs by starting pitcher Wade Miley and Eric Chavez to take a 4-2 lead late into the contest. "There's just something about this team."
Torres had led off the ninth with a single, his second hit of the night. He was sacrificed to second by Brandon Crawford, setting up a lefty-against-lefty matchup, the type Giants manager Bruce Bochy has avoided in his starting lineups with Belt the past two games.
But the club's regular first baseman was told the non-starts were not a benching. Rather, they were a chance to relax and work out the kinks in his .183-hitting swing. And that's exactly what he did Monday.
"Today I did a lot more stuff," he said of his pregame routine. "When I went out there for B.P. (batting practice), I felt I was starting to get it. I just tried to repeat it over and over again.
"When I went to the plate, I felt really comfortable and confident."
Moments later, he wasn't feeling quite as good, the result of a dogpile near second base after the Giants' second walk-off win in the last four games.
"I'm pretty light-headed and my kidneys hurt," Belt admitted afterward. "Somebody got me good."
Sergio Romo (1-1), who retired the only batter he faced -- pinch-hitter Eric Hinske -- to end the top of the ninth inning with the potential go-ahead run at second base, got the win for the Giants, who are now 4-0 on their current six-game homestand.
Neither starter, Arizona's Miley and San Francisco's Ryan Vogelsong, factored in the decision, which extended the Giants' home winning streak to seven games while also giving them a sixth come-from-behind victory of the young season.
Miley, who took a shutout into the seventh inning against the New York Yankees in his previous start, surrendered three doubles and two runs to the Giants' first four batters before settling down and not allowing the home team to add to its total before turning things over to the Diamondbacks bullpen in the seventh as Arizona (10-9) attempted to move within a half-game of San Francisco in the standings.
But Posey feasted on a fastball by Arizona's second reliever, David Hernandez, and took it over the 399-foot mark in straightaway center field with Pablo Sandoval aboard in the eighth to erase a 4-2 Giants deficit and set up the exciting finish.
The home run was the second in two days for Posey, who hadn't homered in the Giants' first 18 games.
"I started feeling good in Milwaukee," Posey said of the Giants' series a week ago. "I've felt like I've had a pretty good approach (since then). I just got a good pitch and hit it. I'm happy it went out."
The home runs by Miley in the fifth inning and Chavez in the sixth, coupled with key offensive and defensive plays by former Giants outfielder Cody Ross, had created the two-run lead Arizona took into the eighth inning.
Miley shocked the sellout crowd at AT&T Park with a blast into the right-field pavilion to break a 2-2 tie in the fifth. It was the first career homer for last year's National League Rookie of the Year runner-up.
"That's just a bonus. Pitchers aren't supposed to hit," admitted the left-handed hitter, who said he hadn't homered since high school in Louisiana. "It was a little bit shocking, especially being the first time I've done that."
Miley was confident the Arizona bullpen would save what would have been his third win.
"They've done a great job. You have all the confidence in the world in those guys," he said of a group that had allowed just five runs in its previous 32 innings. "That's baseball. If we're in the same situation (Tuesday), you'll still have faith in your bullpen."
Chavez gave the lefty another run of breathing room in the sixth when he went the opposite way to left field on a backdoor curve ball for his second homer of the season and a 4-2 lead.
Like Miley, Chavez felt good about taking a late lead, especially with the team having already won four one-run games this season.
"That's a tough loss for us," he admitted. "That's not the way you want to end it, but that's kinda the way you want to draw it up with David (Hernandez) out there. But Buster hit the big home run."
NOTES: Belt's most recent walk-off hit before Monday's came in 2010 in Double-A ball in Richmond. ... Ross was in line to be the hero of the night with a two-run single in the first inning and a lead-saving diving catch near the right-field foul pole on Posey in the sixth. ... Miley, who'd been hitless in five at-bats this season before hitting his home run, had an eight-game hitting streak last season in which he went 9-for-20. ... The Giants are four games into a stretch of 15 consecutive games against NL West competition. They already are 9-1 record against division foes. ... Giants first-base coach Roberto Kelly will miss the entire Arizona series as he attends a graduation in Panama. Special assistant Shawon Dunston coached first base Monday on a "trial" basis.