SAN DIEGO -- San Francisco Giants reliever Santiago Casilla stumbled while delivering a pitch to balk in the winning run as the San Diego Padres won 7-6 in 10 innings on Saturday night at Petco Park.

Adam Rosales tied the game with a RBI single before Casilla (1-3) committed the miscue.

Buster Posey's 10th-inning home run, the Giants' fourth of the game, had given San Francisco its first lead of the game.

Kevin Quackenbush (6-3) got the win despite allowing Posey's homer.

The Padres wasted a late opportunity when Albert Suarez walked Wil Myers to open the seventh and he moved to third on Matt Kemp's single. But Myers was erased trying to score on a grounder, and after an intentional walk to Alex Dickerson, Derek Norris struck out.

With the bases loaded, Josh Osich replaced Suarez and got pinch-hitter Adam Rosales to ground out.

The Giants had a chance to break the deadlock in the seventh, with runners on first and second with no outs. But Brad Hand got Posey to ground into a double play and he struck out Brandon Crawford on a slider.

Neither starter saw the sixth inning.

Jeff Samardzija lasted five frames, surrendering five runs (four earned) on five hits.

Luis Perdomo pitched 4 2/3 innings and was charged with three runs and eight hits. He was pulled one out shy of a potential victory, with the Padres leading 5-3.

The Giants tied the game 5-5 in the sixth on Ramiro Pena's first home of the year. Pena's homer was two batters after rookie Mac Williamson also burnt Carlos Villanueva as he delivered a pinch-hit shot, his third this season and fourth of his career.

The Giants chased Perdomo with two outs in the fourth, when Posey smoked a double down the left-field live, driving in Denard Span to pull within 5-3.

Ryan Schimpf's RBI triple off the top of the right-center field fence in the third gave the Padres a 5-2 edge. Alex Dickerson scored after reaching on Ramiro Pena's error. Earlier in the inning, Matt Kemp stroked his 17th homer.

Pagan rallied the Giants with a one-out, two-run homer in the third, making it 3-2. Samardzija singled before Pagan smashed a 2-2 Perdomo offering 410 feet, deep into the right-field seats. It was Pagan's fifth homer of the season and his second in his last 30 at-bats.

The Padres pushed ahead 3-0 in the second when Schimpf redirected Samardzija's 95-mph fastball over the left-field fence for his fifth home run of the year.

Samardzija struggled with his command in the opening inning and it cost him. He walked the first two batters, which helped put the Giants in a 2-0 hole.

Travis Jankowski and Wil Myers took free passes and Jankowski scored when Posey's throw to third on Jankowski's steal attempt sailed into left field. Yangervis Solarte followed with a broken-bat blooper into center field, plating Myers.

NOTES: San Diego manager Andy Green said RHP Edwin Jackson will be called up from Triple-A El Paso to start against San Francisco in Sunday's series finale. Jackson replaces LHP Drew Pomeranz in the rotation after Pomeranz was traded to Boston. ... Padres RHP Tyson Ross (shoulder) continues to play catch, but not off the mound after spraining his ankle. ... Giants RHP Matt Cain (hamstring), who was roughed up in his final rehab start Friday, will throw a bullpen session on Sunday. Cain could start Wednesday at Boston. ... San Francisco 2B Joe Panik (concussion) could rejoin the team in New York next weekend against the Yankees.