SAN DIEGO (AP) — Jake Peavy’s first start at Petco Park in six seasons didn’t go as well as he would have liked.

Peavy allowed a grand slam to 37-year-old backup catcher Wil Nieves with two outs in the fourth inning, a drive that helped the San Diego Padres beat the San Francisco Giants 6-4 Sunday.

Peavy (0-1) lasted only four innings, allowing four runs and four hits while striking out five and walking three.

“It was just that one pitch,” Peavy said. “You could go back and look at the game and there it is, the one pitch. I wasn’t sharp by any means in that inning, but we got soft contact. I just wasn’t able to avoid a couple of outs and made a bad pitch to Nieves, obviously.”

The Padres took three of four against the defending World Series champions.

It was Peavy’s first start at Petco since 2009, when he was traded from the Padres to the Chicago White Sox.

“I was comfortable,” said Peavy, the unanimous 2007 NL Cy Young Award winner while with the Padres. “I really didn’t have any nerves. I was just trying to find a way to win. I really didn’t take it all in. Everything is really so different now. But the mound and the setup at home plate, I was comfortable for sure.”

It was the first career slam and just the 10th homer overall for Nieves, who was a 47th-round draft pick of the Padres in 1995. He signed as a minor league free agent in January and won the backup job after Tim Federowicz, obtained from the Dodgers in the Matt Kemp deal, was sidelined following knee surgery.

Making his first start of the season, Nieves homered to left on a 1-1 pitch with two outs in the fourth. Peavy (0-1), whose first start of the season was delayed because of a back injury, loaded the bases on a single by Kemp, a walk to Justin Upton and a one-out single by Will Middlebrooks.

San Diego’s Tyson Ross (1-0) allowed three runs and five hits in six innings, struck out eight and walked four. Craig Kimbrel pitched the ninth for his second save in as many chances.

The Giants had given Peavy a 2-0 lead on Matt Duffy’s RBI single in the second and Brandon Belt’s RBI hit in the third. Belt had missed four games with a strained right groin.

Gregor Blanco’s RBI triple down the right-field line in the sixth pulled the Giants to 4-3 before Alexi Amarista’s RBI triple into the left-field corner in the bottom of the inning gave the Padres a two-run lead.

Buster Posey homered off Dale Thayer with two outs in the seventh, his second, to pull the Giants to 5-4.

TRAINERS ROOM

Giants: 3B Casey McGehee said an MRI showed he has a bone bruise in his left knee. He had a cortisone shot and is expected to be out a few days. “All things considered I feel real fortunate. It’s as good of news as I could hope for,” McGehee said. He said he suffered the injury Monday against Arizona and aggravated it during an at-bat in the first inning Saturday.