SAN FRANCISCO -- Barry Zito no longer knows anybody playing on the other side with Oakland. He's not pitching against pals as he did before, when the pressure mounted in such situations and he fizzled.

"That was difficult," Zito said. "You almost don't want to develop friendships because you might be pitching against them next week."

Zito has settled in to his new scene and outdid his old Oakland team this time, and was backed by Brandon Belt's three RBI in the San Francisco Giants' 8-6 victory against the Athletics in the Bay Bridge Series opener Friday night.

Hector Sanchez had two RBI to back Zito (3-1), who won the 2002 AL Cy Young Award across the bay in Oakland but left for a $126 million, seven-year deal with the Giants before the 2007 season.

Josh Donaldson homered, doubled in a run and hit a three-run double off Steve Edlefsen for the A's, giving Donaldson career bests of three hits and five RBI. But Oakland couldn't overcome a shaky night by starter Jarrod Parker (1-2) and lost their 10th straight in San Francisco's waterfront ballpark dating to 2009.

"This game was very emotional for me," Donaldson said. "I've been grinding all season and things just haven't been going my way. Anything that could go wrong, did go wrong. I feel comfortable here and I'll just continue to work at it. Whenever good things happen sometimes I get excited out there."

Zito matched his season high with five strikeouts in 5 1/3 innings and also drew a bases-loaded walk in the Giants' four-run third inning. The A's walked five that inning as San Francisco batted around on the way to scoring its most runs at home since getting eight last Sept. 12 against the Padres. Zito, who improved to only 2-4 with a 7.16 in six starts against the A's, walked off to a warm ovation from the sellout crowd of 41,477 with one out in the sixth after allowing Jemile Weeks' leadoff triple and an RBI single to Cliff Pennington.

Belt had an RBI single in the second, drew a bases-loaded walk in the third that chased Parker, and hit a sacrifice fly in the fourth.

Zito came in with the lowest ERA of San Francisco's starters at 2.53 but raised it to 3.00 and fell behind Matt Cain's 2.67. He was furious to come out of the game.

"He's been solid," said manager Bruce Bochy, who in hindsight wished he had left Zito in the game. "It's nice to see what Z's doing. He's pitching with a lot of confidence."

This has been a comeback season for the 34-year-old lefty, who was kept off the roster for all three playoff rounds during the Giants' run to the 2010 World Series title.

Santiago Casilla, San Francisco's fifth reliever, pitched out of a jam in the ninth for his 10th save in 11 chances. He retired Weeks on a groundout with runners on the corners to end it.

Belt put the Giants ahead 1-0 on an RBI single in the second that scored Angel Pagan, who drew a leadoff walk and stole second. Pagan, coming from second, made a headfirst slide into home but easily beat the throw.

A's pitchers went 0 for 18 with seven strikeouts at the plate last season in interleague play and Parker struck out in the lone plate appearance Friday, extending the pitchers' hitting funk to 0 for 22 dating to Ben Sheets' single on June 19, 2010, at St. Louis.

"Oh, I didn't realize that," Oakland manager Bob Melvin said. "Hopefully we'll be able to pick up on that."

San Francisco's longest home winning streak against an opponent at AT&T Park is 12 games, against the Mets from May 1, 2000-May 16, 2003 -- so the Giants could match that with a weekend sweep. They have a nine-game home winning streak against the Texas Rangers, who visit AT&T Park from June 8-10 still seeking their first victory here.

A special logo was painted on the grass behind home plate in honor of next week's 75th anniversary of the Golden Gate Bridge, and CEO and team president Larry Baer searched before the game for his Golden Gate Bridge neck tie.

Players wore special patches on their jerseys for the occasion, too.

Joaquin Arias had a sacrifice fly, returning to San Francisco's lineup after missing two games since being hit in the left forearm by a pitch Tuesday.

A's lefty Travis Blackley, claimed off waivers this week from the Giants, entered the ballpark on his old San Francisco ID and pitched a 1-2-3 eighth against his old team.

Notes

• Giants C Buster Posey had a planned night off and Sanchez caught Zito as has been customary.
• RHP Brad Penny signed a minor-league deal with the Giants.
• A's slugger and CF Yoenis Cespedes was set to have his injured left hand examined by a team doctor to determine whether he can start swinging.
• Melvin said that he appreciated receiving a call of apology from Thursday's home plate umpire, Laz Diaz, who ejected the manager for arguing a call on a suicide squeeze.
• Melvin went with Donaldson at 3B a day after he had an error that nearly cost Oakland its 5-4, 10-inning victory at Texas.
• The Giants are in a stretch with 20 consecutive games before their next day off May 31.
• A Zito No. 75 signed orange jersey was donated for charity in commemoration of the bridge anniversary
• A's OF Coco Crisp, on the DL with an inner ear infection, did baseball activities and took batting practice. He is expected to do more Saturday and perhaps be activated as soon as Sunday.