SAN FRANCISCO -- Chase Headley had scoring on his mind. Adrian Gonzalez wanted to make sure where the ball was. Headley had the better angle, but Gonzalez had to decipher player movements along with ball trajectory.

Edgar Renteria's fourth-inning grand slam against Jake Peavy made it all academic.

"It goes to show you how one pitch can change the complexion of a game," San Diego manager Bud Black said after the Padres dropped an 8-3 decision to the San Francisco Giants on Tuesday night.

One pitch for Peavy has been the tale of the season.

"It's been a recurring theme for me in my starts," the former National League Cy Young Award winner said. "I feel good in my first start and if I take one pitch back [a two-out, first-inning, bases-loaded single by Los Angeles Dodger James Loney], it's two fewer runs. And the one pitch to [Carlos] Delgado [a three-run home run in the first inning] in my last start."

It was a rare night for Peavy overall. He allowed six earned runs for the first time since his Wild Card playoff start at Colorado in 2007. He threw 85 pitches or less for the 14th time in 203 Major League starts, and just the fifth time since the start of the 2006 season.

His 13th start in San Francisco proved unlucky in another way: He allowed the first grand slam of his notable career in the fourth inning to Edgar Renteria. According to the Elias Sports Bureau, it was the second-longest active streak among pitchers who had never allowed a grand slam (1,284 1/3 innings). Houston Astros pitcher Brian Moehler has the longest active streak among pitchers who have never given up a slam at 1,360.

"You just can't make a mistake like that, especially with the pitcher on deck," Peavy said. "Edgar has good numbers against me, I know that. It's always singles. I know what I have to do to get him out, but you have to execute. If you don't execute, he's going to make you pay."

Giants starter Matt Cain wasn't exactly at his best, but he was good enough. He allowed two runs on nine hits and did not walk a batter. He struck out five.

"He's got great stuff," Headley said of Cain. "He has the capability of making a pitch to get out of a situation. Having said that, we swung the bats well against him; we just couldn't punch anyone in. I have no problem with what we did. We got ourselves in good situations."

The fourth inning turned out to be San Diego's undoing on both ends. In the top of the inning, Gonzalez and Headley each singled to open things up.

Kevin Kouzmanoff drove a ball to the gap in right-center. Gonzalez held to see if the ball would be caught, but Headley had the better angle from first and caught up to Gonzalez.

"I think we both did the right thing," Headley said. "Right off the bat, I knew it wasn't going to be caught. I was at second and Adrian was at second making sure. Once I got close to him, he took off. It was a matter of a split-second. If it's not a perfect play, we both score. Credit [Aaron] Rowand for being in position to play the carom and get the ball in quickly."

When the ball landed, Headley was right behind Gonzalez, and the relay throw was on target at home. Gonzalez was able to just avoid Bengie Molina's tag, but Headley was caught trying to jump over the tag.

"It's just a reaction thing," Headley said. "I wasn't really thinking; I was trying to score."

Kouzmanoff reached third on the play, but was stranded there as Henry Blanco struck out and Luis Rodriguez flied out.

"We got a bunch of singles and Kouz gets a big hit with the ball in the gap and the inning looks like it has the making of a few runs," Black said. "We never got the knockout blow. With a ball hit that high and where Adrian was, it was tough to tell where the ball was. That was unfortunate for us."

The Padres lost their second straight for the first time this season. It follows a lengthy road trip on the East Coast and Monday's rainout in Philadelphia.

"I'm going to get better," Peavy said. "I'm not that far off health-wise."