SAN FRANCISCO (AP) - Josh Donaldson is starting to show signs that he might be a reliable replacement for Scott Sizemore.

Donaldson hit two homers and drove in five runs against Yusmeiro Petit, and the Oakland Athletics avoided a Bay Bridge Series sweep with an 8-1 victory over the San Francisco Giants in Wednesday's spring finale.

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``I've felt good about where he is in his progress, and I think today was a fitting end for him and how much work he's done this spring,' A's manager Bob Melvin said. ``It's all starting to come together right now.'

Donaldson never figured to have such an eventful year.

That all changed in an instant.

Sizemore tore a ligament in his left knee during Oakland's first full-squad workout of spring training, ending his season and making Donaldson - a converted catcher - the starting third baseman before a game had been played. Donaldson finished 4 for 5 in the finale, pushing his batting average up to .270 after a dismal start.

``Every day in spring training I'm in there doing early work, whether it's hitting, defense, then playing the game, then doing more after the game,' Donaldson said. ``Right now, I feel it's easier on me mentally and physically just going out there playing and hitting.'

An otherwise quiet spring ending for San Francisco still had a bright spot.

Buster Posey, receiving rousing ovations from fans every time his name was announced, cleared another milestone with an RBI single in the first. Both were firsts at AT&T Park for the 2010 NL Rookie of the Year since a brutal season-ending collision at home plate with Florida's Scott Cousins last year.

Posey was 0 for 2 with two walks in Monday night's exhibition in San Francisco. He also added a double in the fourth in the last spring training game, setting the stage for San Francisco's chase to reclaim the World Series title the franchise won two years ago.

``We're set now. Now it's time to get the season started,' Giants manager Bruce Bochy said. ``I like the way the spring went. We worked hard and played hard. I think they're ready.'

A sun-soaked day at San Francisco's cozy waterfront ballpark still belonged to an unproven slugger from across the bay.

After a so-so start to spring training, Donaldson smacked a two-run shot to left in the second to put Oakland ahead 2-1. He followed with a three-run homer to deep left-center in the third for his only long balls this spring.

Donaldson's latest go-around in the Bay Area figures to last a little longer than his first.

Originally a first-round draft pick of the Chicago Cubs, Donaldson was traded to Oakland in a deal for pitcher Rich Harden in 2008. He briefly played for the A's in 2010 and hadn't done much since.

Sizemore was acquired by Oakland last May in a trade with Detroit. He hit .249 with 11 home runs and 52 RBIs in 93 games with Oakland.

The 26-year-old Donaldson wasn't the only A's hitter to suddenly find some pop in a lineup that rarely shows any of it.

Jonny Gomes - from nearby Petaluma - also hit a solo home run earlier in the third, helping Oakland build an 8-1 lead.

Brian Wilson retired both batters he faced in the eighth, all the work Bochy wanted from his bearded closer in the exhibition finale.

The Giants open the regular season at defending NL West champion Arizona on Friday. The A's host Seattle in the last two of a four-game set that began in Japan and was squeezed between the final week of the exhibition schedule.

Not that Oakland ever discounts a win at the home of the rival Giants.

``We needed to create something going into the season,' Melvin said. ``Momentum for a new group, confidence for a new group, I think, that type of winning in spring training can carryover.'

NOTES: A's reliever Joey Devine was still waiting to hear the results from an MRI on his surgically repaired right elbow. Dr. James Andrews was scheduled to read the scan for Devine, who had Tommy John surgery in April 2009 and is experiencing more problems. Devine is already on the DL with right biceps tendinitis and has been shut down for now. He hopes this won't threaten his season. ``This is the story of my life,' Devine said. ``It's just a constant. It's pretty sore - same symptoms, putting shampoo in my hair, turning a doorknob.' ... The A's optioned RHP Tyson Ross to Triple-A Sacramento. Melvin plans to have Ross return April 17, the first time the A's will need a fifth starter. ... The Giants will open AT&T Park for a free viewing party Friday night so fans can watch the team's opener at Arizona.