With runners on the corners, Amarista hit the ball to right-center field off of relief pitcher Josh Osich, scoring Travis Jankowski, who came in as a pinch-runner after Derek Norris hit a leadoff double to spark the ninth-inning rally that led to a 5-4 win.
Matt Kemp notched his 99th RBI of the season, as he hit a double to left field in the eighth inning, scoring Cory Spangenberg to tie the game at 4-4.
The Giants, who can be eliminated from the National League West race as early as Saturday, amassed eight hits in the first three innings but scored just two runs in that span.
"We squandered a lot of opportunities," said Giants manager Bruce Bochy, who also lamented the leadoff walk Hunter Strickland issued to Wil Myers that helped San Diego pull even in the eighth. San Francisco went 3-for-17 with runners in scoring position.
MOMENTS THAT MATTERED Hang it and bang it: Norris jumped on a 75 mph curveball on the outside corner in the second inning from Madison Bumgarner, hitting a 385-foot home run to right-center field. It was Norris' first home run since Aug. 14 and the first time any of his 40 career home runs have gone to the opposite field.
Start us up: Giants leadoff batters reached base safely in the first five innings and six of the first seven. That impetus proved valuable, as four of those hitters scored. Angel Pagan did the honors twice, slicing doubles to left field in the first and third innings.
You're out: In the first inning, Alejandro De Aza hit a single which looked like it would score Pagan. However, a throw from center fielder Melvin Upton Jr. to Myers was relayed to Norris in time to get Pagan out at the plate. The 8-3-2 play saved a run early turned out to be crucial for the Padres.
Bummed out: Bumgarner left the game after seven innings with a 4-3 lead and a chance at securing his 19th victory -- tantalizingly close to a coveted 20th win. But Kemp's aforementioned hit denied Bumgarner the decision. The left-hander nevertheless performed impressively, surrendering three hits and striking out nine. Bumgarner still could win 20, since the Giants can assign him as many as two more starts with 10 games remaining. More >
"That was beautiful. [Hitting coach] Mark Kotsay deserves the credit. He recommended Lexi. He liked the matchup with Osich." -- Padres interim manager Pat Murphy on Kotsay's decision to tab Amarista to pinch-hit in the ninth inning
SOUND SMART WITH YOUR FRIENDS
It's been a strange season for Ian Kennedy, who had a 6.40 ERA in six starts in May and then had a 2.63 ERA in a 16-start stretch in June, July and August. However, after getting a no-decision Thursday, he has a 6.51 ERA in five starts this month with likely one start left in the season and likely his career with the Padres. Kennedy will be a free agent after the season.
NORRIS STRAINS GROIN
Padres reliever Bud Norris left the game in the top of the sixth inning after throwing 11 pitches with what the team said was a mild right groin strain. He got Kevin Frandsen to fly out and then allowed a single to Bumgarner before leaving the game. He was replaced by Nick Vincent.
"I over-strided and felt it grab real quick," Norris said. "I appreciate Murph getting me out of there quickly. Hopefully, it feels better tomorrow."
In the ninth inning with the game knotted at 4-4, Bochy challenged a call that Pagan was out after stealing second base. After reviewing the play, the umpires overturned the call.
CALL ME BABY
It was a wild night off the field, too, as a baby was born at Petco Park for the first time in its history since opening in 2004. A baby boy, Levi, was born in the early innings of the game and delivered by a Padres employee, registered nurse and midwife, Donna Borowy. "This is the youngest Petco fan we'll ever have," Borowy said afterwards.
WHAT'S NEXT Giants: San Francisco will renew its Interleague rivalry against the Oakland A's with a 7:05 p.m. PT encounter at the Coliseum. Giants right-hander Mike Leake will oppose Oakland ace Sonny Gray. Each starter has faced the other team only once in his career.
Padres:Casey Kelly (0-0, 4.50) makes his first Major League start since Sept. 27, 2012 when he starts the opening game of a three-game series against the D-backs at 7:10 p.m. PT at Petco Park. Kelly, who had Tommy John surgery in 2013, made a relief appearance against the Rockies last Saturday and allowed one earned run in two innings.
Chris Haft is a reporter for MLB.com. Read his blog, Haft-Baked Ideas, follow him on Twitter at @sfgiantsbeat and listen to his podcast. Beth Maiman is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.