OAKLAND, Calif. - When Byung-Hyun Kim blows a postseason assignment, it's no surprise. When the Oakland Athletics win with a bunt, it's practically unprecedented.

Ramon Hernandez dropped a perfect bases-loaded bunt with two outs in the 12th inning, scoring Eric Chavez with the winning run to lift the A's over the Boston Red Sox 5-4 in their playoff opener Wednesday night.

At 4 hours, 37 minutes, the game was the longest in Oakland's postseason history, and it ended in the most improbable way: with the hard-hitting A's executing a small-ball play to eke out a run.

By the time Chavez crossed the plate shortly after 11:45 p.m. PDT, the teams were less than 14 hours away from the start of Game 2 on Thursday.

Chavez helped prevent Boston from taking the lead in the top of the 12th, making a tremendous play at third base.

Todd Walker homered twice and had four hits, putting Boston in position to pull off a win in the AL's toughest road ballpark. Then came the latest postseason misadventure for Kim, whose struggles nearly cost Arizona its World Series title two years ago.

In the ninth, Kim walked a batter and hit another before Erubiel Durazo drove home pinch-runner Eric Byrnes with a tying two-out single off Alan Embree.

In the 11th, Boston manager Grady Little showed just how much he trusts his bullpen by calling on Derek Lowe, his scheduled starter in Game 3 on Saturday.

Lowe got into trouble in the 12th, walking three batters to load the bases. Hernandez, the A's All-Star catcher, dropped an exceptional bunt down the third-base line, and Chavez scored without a throw.

After Keith Foulke pitched three innings of scoreless relief for the A's, they went to rookie Rich Harden. He walked two batters and threw a wild pitch in the 12th, but Chavez saved the inning by fielding Gabe Kapler's sharp grounder and diving to tag third base before Manny Ramirez got there.

Harden, called up to the majors after the All-Star break, got the win.

Durazo, who had an early two-run double against Pedro Martinez, tied it with a clean single to center that delighted most of the 50,606 fans in the packed Coliseum.

The Red Sox, who added another heartbreaking playoff defeat to the star-crossed franchise's overflowing collection. Boston has lost seven of its last eight playoff openers.

Martinez and Walker were the stars of the first seven innings. Martinez remained unbeaten in his playoff career by narrowly outpitching Tim Hudson, but he didn't get the win.

Martinez yielded six hits and four walks in seven innings, throwing a season-high 130 pitches. But Boston's imposing ace was never far from trouble: He allowed three runs in the third, threw out a runner at home in the fifth and barely escaped a bases-loaded jam in the seventh.

Until Kim and Embree blew it, Boston's heavy-hitting lineup appeared headed to a win thanks to two big blows from one of its lightest bats.

Walker, a well-traveled second baseman in his first year with the Red Sox, hit a solo homer in his first career playoff at-bat. After two singles in the middle innings, he hit a two-run homer off reliever Ricardo Rincon in the seventh, giving Boston a 4-3 lead.

Walker had the eighth multihomer game in the long postseason history of the Red Sox, who are in their 85th season since their last World Series championship.

Jason Varitek also homered and reached base four times for the Red Sox, who tagged Hudson for 10 hits and three runs.

The damage could have been worse, but Ramirez stranded five runners while going 0-for-5.

Though he also escaped several jams, Hudson's postseason struggles continued: He has just one victory in six career playoff starts. The right-hander appeared to be pitching through pain in his final two innings, wincing after some pitches and drawing the concern of Oakland's medical staff.

Durazo got the biggest hit in Oakland's third-inning rally, driving home Chris Singleton and Ellis with a hit into the right-center gap. Miguel Tejada, who batted .143 in the division series last season, followed with a single up the middle.