Dan Haren allowed three hits over seven dominant innings, and Cust hit a three-run homer in the A's second straight victory over Bonds' cross-water crew, 4-2 on Saturday night.
During his years of minor-league toil and major-league false starts, Cust looked up to only a few hitters -- and Bonds' combination of patience and power was at the apex of his aspirations. He resisted the Baltimore Orioles' attempts to turn him into a free swinger, but kept the quick cut that allowed him to crush Matt Cain's stray fastball in the first inning.
"I love watching (Bonds') approach," said Cust, who also drew two walks to give him 16 with Oakland. "He's been the guy forever. He's not sneaking up on anybody, but he's got kind of the best approach ever."
Since joining the A's on May 4 after a minor-league trade with San Diego, Cust has been even more powerful than the greatest active power hitter, clubbing eight homers in 13 charmed games -- including the blast that beat the Giants.
"What a great find," A's manager Bob Geren said. "He's on a very hot streak. His strike zone is just very consistent every night, and whenever you can keep doing that, you have a chance to do something special."
While Bonds played in his ninth straight game without hitting a homer, two of the Bay Area's top young pitchers showed off in front of another Coliseum crowd with divided loyalties -- and Cust's early shot held up.
Haren (4-2) retired 15 straight batters after a rocky 33-pitch first inning, giving up his only runs on Bengie Molina's two-run homer in the seventh. Three relievers finished it off, with Alan Embree pitching the ninth for his first save with the A's.
"You can't ask for three runs in the first inning every time out, but getting it every once in a while is great," said Haren, who struck out six. "After throwing 33 pitches in the first inning, I had to be more aggressive or I wasn't going to last in the game."
But Cain (2-4) nearly matched Haren, striking out nine and yielding just six hits in 6 1/3 innings. Cain, who threw a one-hitter at the Coliseum last May, walked Nick Swisher and gave up a two-out single to Dan Johnson in the first before Cust's shot.
"That's the guy I don't want to beat me, and I gave him the chance to do just that," said Cain, who has lost three of his last four starts despite solid numbers. "I messed up. That's a ball I normally get every time, and I just missed it. That's my fault right there."
Bonds went 1-for-3 with a single, but the slugger hasn't added to his 745 career homers since May 8. San Francisco, which walked 13 in a 15-3 loss to the A's on Friday night, has lost eight of 11, including four of its last five while falling back below .500.
"When we're not clicking offensively, one pitch is the difference in the game," San Francisco manager Bruce Bochy said.
Haren hasn't allowed more than three earned runs in any of his AL-leading 10 starts while pitching at least five innings in every trip to the mound -- but the right-hander got no decision in his three previous starts.
He walked two and barely escaped the first inning on a bases-loaded grounder by Molina, but quickly found a groove. Just two of the next 15 hitters even got the ball into the outfield before Bonds lined a leadoff single to right in the seventh.
But Molina popped his fourth homer into the left-field stands, and Haren subsequently threw two wild pitches before escaping the inning on a groundout.
Jason Kendall drew a bases-loaded walk in the eighth for Oakland's final run.
- Randy Winn led off the game with a single to extend his hitting streak to 18 games. Winn's streak is the Giants' longest since Robby Thompson hit in 21 straight in 1993.
- Swisher had his hair cut by his father behind home plate before the game. His lengthy locks will be donated to a charity that makes wigs for cancer patients. After the game, he got a real haircut from a stylist in the Oakland Raiders' locker room.
- Cust punctuated his homer with an old-fashioned "Bash Brothers" forearm high-five with Swisher and Johnson.