BOSTON (Ticker) -- David Ortiz won another Boston marathon.

Ortiz hit a two-run homer in the bottom of the 12th inning and the Boston Red Sox avoided a sweep in the American League Championship Series with a dramatic 6-4 victory over the New York Yankees .

Nine days after his 10th-inning home run clinched the AL Division Series against Anaheim, Ortiz deposited a 2-1 fastball from Paul Quantrill (1-1) into the Yankees' bullpen beyond the right field fence to send Fenway Park into bedlam.

Ortiz tossed away his helmet as he headed toward home plate, then leaped into a scrum of waiting teammates. The blast ended the longest game in ALCS history and forced a fifth game here Monday.

"Pretty much every time I've faced him, he makes good pitches on me, especially with the fastball coming right at me and going back to the plate," Ortiz said. "I guess it's a good pitch from a righthander to a lefthander because once it's coming, you give up. And I was kind of looking for it."

"I made a good pitch to Manny (Ramirez) that he fouled off, and that forced me to go after Ortiz differently than I would have liked to," said Quantrill, who gave up a leadoff single to Ramirez. "The bottom line is that I just didn't get my job done."

The contest lasted 5 hours, 2 minutes and gave the teams little time to prepare for Game Five, which begins at 5:10 p.m. EDT.

Game One winner Mike Mussina faces Game Two loser Pedro Martinez .

"I've always felt since I started doing this thing about nine years ago that momentum is easily changed in a short series," Yankees manager Joe Torre said. "You just have to make sure that when we go out there for the next game that we can't carry this baggage with us."

Boston was three outs away from an ignominious sweep at the hands of its most bitter rival before rallying against vaunted closer Mariano Rivera in the bottom of the ninth.

Kevin Millar drew a leadoff walk and pinch runner Dave Roberts immediately stole second before racing home on Bill Mueller 's single up the middle.

On July 24, Mueller's two-run homer off Rivera in the bottom of the ninth gave the Red Sox a wild 11-10 win over the Yankees.

"He's just the perfect guy to have on your club," Red Sox manager Terry Francona said of the 2003 AL batting champion. "He gives you everything you ask."

This was just the fourth blown save in 36 postseason chances for Rivera, who had not allowed a run in his first 9 1/3 innings in this year's playoffs.

"We're so used to Mo going out there and getting people out, which he did tonight," Yankees manager Joe Torre said. "It's just that the walk and the stolen base (were) the difference in that ninth inning."

"You battle Mariano Rivera ," Millar said, "you don't ever have his number."

One night after getting pounded in a record-setting 19-8 loss to the Yankees, the Red Sox bullpen performed valiantly. Mike Timlin gave up one run in one inning, but Keith Foulke , Alan Embree , Mike Myers and Curtis Leskanic combined for 5 2/3 scoreless frames.

Leskanic (1-0) got the last four outs for his first career postseason win.

The mood at Fenway Park was decidedly different some four hours earlier, when Alex Rodriguez hit a two-run homer off starter Derek Lowe to put the Yankees in front 2-0 in the top of the third.

Boston rallied two innings later for just its second lead of the series. New York's Orlando Hernandez cruised into the fifth in his first postseason start before encountering control problems.

Walks to Millar and Mark Bellhorn put runners at first and second. Orlando Cabrera delivered a two-out RBI single, Ramirez walked and Ortiz came through with a two-run base hit to put the Red Sox in front, 3-2.

Just like Game Three, however, the lead did not last long.

Lowe surrendered a one-out triple to red-hot Hideki Matsui and was replaced by Timlin. Bernie Williams singled to tie it and Jorge Posada walked, but Williams was thrown out at third after a pitch trickled away from catcher Jason Varitek .

Ruben Sierra reached on an infield hit and Tony Clark singled off the glove of second baseman Bellhorn to give the Yankees a 4-3 lead.