Peavy won the NL Cy Young Award last year but this difficult season can't end soon enough.
"It's disappointing without a doubt," he said. "I haven't been good enough to win for whatever the reason."
Many of Peavy's numbers are comparable to his sensational season last year when he led the majors in ERA (2.54), topped the NL with 240 strikeouts and finished with a 19-6 record.
"It's just frustrating," Peavy said. "The whole season has been frustrating, personally and for the team."
With San Diego employing a six-man rotation, Peavy (9-11) is scheduled to get just two more starts. Even if he wins his final two outings, Peavy will finish with a non-winning record for the second time in three seasons. He went 11-14 in 2006.
"He's pitching in a ton of low-scoring games and we are not coming out on top of those," Padres manager Bud Black said.
Peavy came in with the third-worst run support in the NL at 3.53 runs per game, and just 2.7 runs at home. The Padres have scored only five runs while he has been in the game in his last four starts.
Against the Giants, Peavy allowed five runs and tied his career high by giving up 11 hits in eight innings. It was the first time in 14 home starts that Peavy has allowed more than three earned runs.
"I didn't feel very good at all," he said. "I was battling my pitches. They hit some balls hard and when they didn't hit it hard, they found holes."
Ishikawa hit a solo drive in the sixth to give the Giants a 3-2 lead. It was his third homer of the season. Sandoval connected in the fourth and Randy Winn singled in a run in the fifth to give San Francisco a 2-0 lead.
"I tried to get my pitch and he left it up," Sandoval said. "He's one of the best pitchers in the big leagues right now."
Jonathan Sanchez (9-10) allowed two runs and six hits in six-plus innings, ending a nine-start winless streak with his first victory since he beat Oakland on June 29.
Brian Wilson pitched the ninth for his 38th save in 43 chances after two straight blown saves.
Peavy's ERA increased from 2.63 to 2.71, dropping into third place in the NL behind New York's Johan Santana (2.70). San Francisco's Tim Lincecum, who is scheduled to pitch on Saturday night, leads the NL at 2.54.
Peavy still leads the majors with a 1.73 home ERA.
Ishikawa went 3-for-3 with a walk and scored two runs. Winn also had three hits for the Giants.
"Great job by the kids going against one of the best," San Francisco manager Bruce Bochy said.
Adrian Gonzalez went deep for San Diego, hitting his third home run in the last two games. His two-run shot in the fifth tied it at 2, and extended his career highs to 32 homers and 105 RBIs.
The Giants have won six of seven.
Peavy, who allowed two home runs for only the second time in 26 starts, has given up 11 hits three times in his career. ... Peavy went 2-for-3, his second multihit game of the season. ... The Giants are tied with the Los Angeles Dodgers for the best record in the NL West since Aug. 1 at 22-18. ... Padres CF Chip Ambres was removed from the game after the fourth inning with a sprained left knee. Ambres tweaked his knee when he beat out a grounder in the first inning.