SAN DIEGO (AP) - Kirk Gibson can't wait to get off the road and back to Arizona. Problem is his Diamondbacks are slowly seeing their playoff chances disappear.

Chase Headley hit a grand slam to extend his career bests to 27 homers and an NL-high 102 RBIs, and Carlos Quentin followed with a homer for San Diego's first back-to-back shots this season as the Padres beat the Arizona Diamondbacks 8-2 Sunday.

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The Padres took two of three in the series and won for the eighth time in nine games against the Diamondbacks. The Padres have won 14 of 19 overall and 18 of 24 at home.

Arizona, the defending NL West champions, dropped 6 1/2 games behind St. Louis for the second NL wild-card spot, with three teams in between them.

After winning twice at Los Angeles to open this 10-game trip, the Diamondbacks dropped five of the next eight.

``It's disappointing,' Gibson said. ``We've lost a lot of close games. We have been close to a .500 club most of the later part of this year and it hasn't been good enough.'

Batting left-handed, Headley continued his breakout season when he drove a 0-1 pitch from Takashi Saito several rows into the right-field stands with two outs in the seventh to give the Padres a 7-1 lead. Knowing he got all of it, Headley briefly watched his shot sail out before beginning his trot. It was his second grand slam this season and third of his career.

It is the Padres' first 100-RBI season since Adrian Gonzalez had 101 in 2010 and the 18th in club history.

John Baker drew a one-out walk and Chris Denorfia kept the inning alive when he reached on second baseman Aaron Hill's two-out fielding error, which made all five runs that inning unearned. Logan Forsythe walked ahead of Headley's slam.

Headley homered twice in Friday night's win against the Diamondbacks, once from each side of the plate, and drove in four runs. His 19 homers and 60 RBIs since the All-Star break lead the big leagues. Overall, his 15 games of three or more RBIs lead the majors.

``The two games we lost here, Headley hurt us,' Gibson said.

Headley's previous highs were 12 homers and 64 RBIs in 2009.

Quentin followed Headley's slam with a shot into the stands in left, his 16th, to chase Saito.

Chris Denorfia homered leading off the sixth, his seventh of the season.

Rookie Andrew Werner (2-1), making his fourth career start, allowed one run and four hits in 6 1-3 innings, struck out six and walked three.

The Padres jumped on Arizona rookie Patrick Corbin (5-7) for two runs in the second. Quentin hit a leadoff double, Jesus Guzman walked, Cameron Maybin doubled over leaping third baseman Chris Johnson and Andy Parrino hit a sacrifice fly.

Arizona's Jason Kubel hit an RBI grounder in the third.

Corbin allowed three runs and six hits in 5 1-3 innings, struck out three and walked two.

``Patrick did fine,' Gibson said. ``He made a couple of mistakes. Overall, a pretty solid performance.'

NOTES: Diamondbacks rookie CF Adam Eaton doubled San Diego's Andy Parrino off first base on the fly. John Baker's fly ball looked like it would go over Eaton's head but the fielder tracked it down, stopped, turned around and threw all the way to first. ... The Diamondbacks are off Monday before opening a two-game series at home against the Los Angeles Dodgers on Tuesday night. The scheduled starters are Arizona's Ian Kennedy (12-11, 4.39) against L.A.'s Joe Blanton (9-12, 4.92). The Padres continue their homestand by opening a three-game series Monday night against the St. Louis Cardinals. The scheduled matchup of lefties will have St. Louis' Jaime Garcia (4-6, 4.16) starting against Eric Stults (5-2, 2.35).