SAN DIEGO (AP) -- Chase Headley put a mighty swing on a fastball from Takashi Saito, watched for a moment as it sailed toward the right-field seats, raised his right arm in celebration and then started his home run trot.

He knew it was gone, and it wasn't just any homer for Headley, the switch-hitting slugger who's having a breakout season for the San Diego Padres.

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.

''It was neat,'' Headley said. ''There's not a lot of balls that I hit here that I know for sure are home runs when I hit them. Yeah, I hit that one pretty good.''

In previous seasons, Headley has been criticized for not putting up the kind of power numbers expected of a third baseman. Now, the switch-hitter seems to be making up for lost time with a more aggressive approach.

Batting left-handed, Headley drove an 0-1 pitch from Saito several rows into the stands with two outs in the seventh to give the Padres a 7-1 lead. 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.

''It was a great feeling,'' Headley said. ''Obviously that's a number along the way that means quite a bit. To be able to share that with my teammates and to get over the hump with a grand slam, just a great feeling.''

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.

Headley had gone 0 for 3 until his slam.

''They're trying to get me out, and that's the great thing about our lineup right now is they can't just sit there and pitch around me because we have other guys who are swinging the bat so well,'' Headley said. ''That's the only way you can have an accomplishment like that is having the rest of the guys doing such a great job around me.''

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

Headley's 60 RBIs in the second half tie him with hitting coach Phil Plantier (1993) for second-most by a Padres player in the second half of a season. That trails only the 81 RBIs that Ken Caminiti had during the second half of 1996, when he won the NL MVP.

Plantier knows how good it felt for Headley to round the bases. In 1993, Plantier homered in the season's penultimate game - his 34th - to reach 100 RBIs on the nose.

''He's a grinder,'' Plantier said. ''He doesn't give a pitch away; he doesn't give an at-bat away. He always gives himself a chance every at-bat of every game, which is hard to do. That feeling he had running around the bases, I'm really happy for him because he's worked extremely hard to do what he's done to this point. The nice thing is, he has three weeks to keep going.''

Said manager Bud Black: ''He's been there for us all year. He's been there especially for us the last 2 1/2 months. Good for him. I'm so happy for him.''

The Padres took two of three 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.

''The two games we lost here, Headley hurt us,'' Arizona manager Kirk Gibson said.

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.

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.

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 had an RBI grounder in the third.

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

NOTES: Diamondbacks rookie CF Adam Eaton doubled San Diego's Andy Parrino off first base. John Baker's fly ball looked like it would go over Eaton's head, but he 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).