SAN DIEGO -- Reserve San Diego Padres catcher Wil Nieves hit a grand slam off former batterymate Jake Peavy Sunday afternoon to lead the Padres to a 6-4 win over the San Francisco Giants at Petco Park.

The win gave the Padres three wins in their home season-opening series against the reigning World Series champions.

Peavy was the pitcher on July 21, 2002, when Nieves made his major league debut.

The Giants were leading 2-0 in the bottom of the fourth when Nieves lined a 1-1 fastball from Peavy 364 feet into the left-ield stands.

The Padres had loaded the bases with one out on a lead-off single by right fielder Matt Kemp, a walk drawn by left fielder Justin Upton and a single by third baseman Will Middlebrooks.

The grand slam was the first of Nieves' career and only the 10th homer of the 37-year-old's 1,153-at-bat career.

The Giants twice reduced the Padres' lead to a single run, in the sixth and seventh. The Padres countered in the bottom half of both innings.

San Francisco right fielder Justin Maxwell walked with one out in the top of the sixth and scored on center fielder Gregor Blanco's triple into the right-field corner to make it 4-3. The Giants missed a chance to tie when third baseman Matt Duffy grounded out to second with one out and the Padres infield drawn in. Padres right-handed starter Tyson Ross struck out Giants relief pitcher Yusmeiro Petit to end the threat.

The Padres stretched their lead to 5-3 in the bottom of the inning on a double by Upton, who has hit safely in every game of the season, and a two-out triple into the left-field corner by shortstop Alexi Amarista.

Giants catcher Buster Posey homered in the top of the seventh -- a 394-foot drive to left center off Padres right-handed reliever Dale Thayer -- to make it 5-4. But Padres center fielder Will Venable countered with his first homer of the season in the bottom of the inning, a 351-foot shot down the line in right off Giants' right-handed reliever Jean Machi.

Ross (1-0) picked up the win for the Padres despite allowing three runs on five hits and four walks in six innings. Ross struck out eight Giants. Right-hander Craig Kimbrel picked up his second save in the series and his second as a Padre.

Peavy suffered the loss in his season debut, allowing four runs on four hits and three walks in four innings. The right-hander missed his first scheduled start last Tuesday due to back stiffness. The start was the first for Peavy at Petco Park since he was traded to the Chicago White Sox on July 31, 2009. Peavy won the Cy Young Award for the Padres in 2007.

The Giants took a 2-0 lead with single runs in the second and third against Ross.

Maxwell walked with one out in the second, advanced to second on Blanco's grounder to third and scored on Duffy's two-out single.

Left fielder Nori Aoki opened the Giants third with a single, extending his hitting streak to 15 straight games dating back to last season. Aoki moved to second on an infield out, advanced to third on a passed ball by Nieves and scored on first baseman Brandon Belt's single.

NOTES: Giants 3B Casey McGehee suffered a bone bruise Saturday night and will miss "a few" games after receiving a cortisone injection. An MRI exam was negative . . . Sunday's crowd of 40,184 raised the total for the series to 168,171 -- a Padres record for a four-game series at Petco Park . . . Jake Peavy's National League Cy Young Award in 2007 came during Bud Black's first season as the Padres' manager . . . RHP Chris Heston will become only the second rookie in San Francisco Giants history to start the home opener Monday. The first was RHP Alan Fowlkes on April 13, 1982 . . . The Padres tied a Petco Park record with their 20-hit game Saturday night.