By Jeremy Kahn

Wil Nieves May have caught Jake Peavy in his major league debut back in 2002 for the San Diego Padres, but he spoiled his return to Petco Park.

Nieves hit his first career grand slam off of Peavy in the bottom of the fourth inning, as the Padres defeated the San Francisco Giants 6-4 in the finale of the four-game series at Petco Park.

With the victory, the Padres won their third straight game and sent the defending World Champion Giants home after a 3-4 opening road trip.

Peavy gave up a single to Matt Kemp, a walk to Justin Upton and a single by Will Middlebrooks with one out that loaded the bases for Nieves.

Peavy, who won the 2007 Cy Young Award in unanimous fashion allowed four runs on four hits in four innings of work, while walking three and striking out five.

Former Bishop O’Dowd High and CAL pitcher Tyson Ross, who also pitched for the Oakland A’s gave up three runs on five hits, while walking four and striking out eight in six innings of work.

The Giants gave Peavy a 2-0 lead entering the bottom of the third inning, as Matt Duffy hit an RBI single in the top of the second inning and Brandon Belt drove in a run in the top of the third inning.

Buster Posey hit his second home run of the season off of Dale Thayer in the top of the seventh Inning.

Wil Venable, the son of former Giants outfielder Max Venable closed out the scoring in the bottom of the seventh Inning, his first home run of the young season.

Casey McGehee, who left Saturday’s game with a knee injury underwent an MRI and it revealed a bone bruise, meaning the third baseman will be back in a few days.

Rookie Chris Heston will make his second start of the season in Monday’s home opener against the Colorado Rockies.

Heston is the first Giants rookie to start a home opener since Alan Fowlkes in 1982.