SAN DIEGO -- With Jedd Gyorko playing shortstop, Alexi Amarista is merely a spare part for the San Diego Padres this month.

Amarista had 16 previous at-bats this month when he waited in the on-deck circle Thursday night with the winning run on third and one out in the bottom of the ninth and left-hander Josh Osich warming up for the San Francisco Giants.

Padres hitting coach Mark Kotsay told the .211-hitting Amarista that Osich was a good matchup, even though both are left-handers.

"I didn't feel any pressure at all," Amarista said. "I was ready. I just wanted a pitch to hit."

He got it on the second pitch and singled homer catcher Derek Norris to give the Padres a second consecutive walk-off win against the Giants, 5-4.

San Francisco fell eight games behind the Los Angeles Dodgers in the National League West with 10 games to play.

"Kotsay called it," Padres interim manager Pat Murphy said. "He thought Alexi was a good matchup. It was beautiful execution by Alexi."

Norris, who hit a three-run homer in the second inning off Giants starter Madison Bumgarner, opened the Padres' ninth with a double to left off San Francisco right-handed reliever Mike Broadway. A sacrifice bunt by center fielder Melvin Upton Jr. moved Norris to third.

Broadway (0-2) then walked pinch hitter Brett Wallace. Osich came into face Amarista.

"It's tough to lose games like this, especially with Bum going for his 19th win," Giants manager Bruce Bochy said.

San Diego closer Craig Kimbrel (4-2) threw one scoreless inning to pick up his second win in as many nights.

The Padres tied the game at 4-4 in the bottom of the eighth when right fielder Matt Kemp doubled home third baseman Cory Spangenberg for his 99th RBI of the season.

The double down the line in left off right-handed reliever Sergio Romo ended Bumgarner's bid for a 19th win.

The San Francisco left-hander retired 18 of the final 21 hitters he faced after Norris' homer. Bumgarner allowed three runs on three hits and three walks over seven innings.

Bumgarner was in line for the win after the Giants rallied from a 3-1 deficit against Padres right-hander Ian Kennedy.

When Bumgarner last faced the Padres on Sept. 12 at AT&T Park in San Francisco, he retired the first 23 hitters he faced until Melvin Upton Jr. singled to center with two outs in the eighth inning.

This time, the bid for perfection ended with the fourth batter. Soon to follow were Bumgarner's hopes for a no-hitter and a shutout.

Padres left fielder Justin Upton drew a walk to open the second. Second baseman Gyorko singled, and Norris followed with a home run to right on a 1-2 pitch, the 381-foot drive putting the Padres ahead 3-1.

Meanwhile, the Giants had plenty of opportunities against Kennedy but still trailed 3-2 after four innings despite having nine hits. In fact, eight of the first 12 hitters Kennedy faced had hits, including a homer and three doubles.

However, center fielder Angel Pagan was thrown out at the plate in the first, and Kennedy coaxed double-play grounders to get out of jams in the second and third.

Shortstop Brandon Crawford led off the second with his 20th home run, a 417-foot shot to center, to give the Giants a short-lived 1-0 lead. Right fielder Marlon Byrd and second baseman Kelby Tomlinson followed with back-to-back singles, but first baseman Kevin Frandsen grounded into a double play and Bumgarner took a disputed called third strike by plate umpire Joe West to end the threat.

The Padres struck for three runs immediately after Bumgarner and West exchanged words.

Pagan opened the third with his second double and scored on a single by left fielder Alejandro De Aza to make it 3-2. Third baseman Matt Duffy doubled to put runners at second and third with no ones. Kennedy struck out catcher Buster Posey, then intentionally walked Crawford before Byrd grounded into an inning-ending double play.

The Giants took a 4-3 lead in the top of the fifth, although it took a balk from Kennedy to get the go-ahead run across.

De Aza opened the fifth with a double and moved to third on a grounder to the right side before Crawford drew a two-out walk. Byrd singled home De Aza with Crawford moving to third, from where he scored when Kennedy balked for the first time this season.

The Giants were 11-for-23 with 13 baserunners in five innings against Kennedy, but they were 3-for-12 with runners in scoring position.

NOTES: 1B Kevin Frandsen made his first start for the Giants since 2009. He finished the regular season with Triple-A Sacramento, and the injury-depleted Giants purchased his contract Wednesday. ... San Francisco 1B Brandon Belt met with a specialist in Pittsburgh on Thursday to discuss his concussion. ... Padres 3B Yangervis Solarte, who came out of Wednesday's game with a right hamstring cramp, was a late scratch from the starting lineup Thursday with the same problem. ... San Diego 1B Wil Myers was back in the leadoff spot Thursday after being scratched Wednesday night with minor discomfort in his surgically repaired left wrist. ... LHP Cory Luebke, who underwent two Tommy John surgeries since he last pitched in a major league game in 2012, is scheduled to face live hitters within the week at the Padres' year-round training base in Peoria, Ariz.