Red Sox left fielder Jason Bay scores the winning run in the ninth inning on a single by Jed Lowrie to beat the Angels and advance to the ALCS against the Rays.

The Providence Journal / Bob Breidenbach

BOSTON — After careening from disaster to euphoria in a span of only minutes, the Boston Red Sox are saying goodbye to a powerhouse and hello to baseball’s underdogs.

The Red Sox rode a superb start from lefty Jon Lester before watching their bullpen cave in and not only give away a two-run lead but allow the Los Angeles Angels to push the go-ahead run to third base in the ninth inning. That’s when some terribly fortuitous luck turned in Boston’s favor and added up to an improbable 3-2, walk-off victory that gave the Sox a 3-1 win in the American League Division Series.

Game One of the ALCS against the surprising Tampa Bay Rays is Friday night in Florida.

The Red Sox were only a play or two away from blowing this game. Their first step back from the grave took place in the top of the ninth inning. With Reggie Willits on third base and one out, reliever Manny Delcarmen was met on the mound by catcher Jason Varitek.

“Tek told me to watch for the bunt. They bunted on me before in L.A.,” Delcarmen said. “I wasn’t sure but I was ready.”

Sure enough, on a 2-1 pitch to Erick Aybar, the Angels chose to send Willits on a suicide squeeze. Aybar missed the bunt attempt and Varitek instantly ran at Willits all the way up the third-base line. Varitek leaped at Willits, slapped a tag on him and, after he hit the ground, the ball squirted loose. However, umpire Tim Welke ruled the ball came loose only after Varitek hit the turf.

“I’ve never saw Tek run so fast to track (Willits) down,” Delcarmen said.

The stunning Angel miscue set up step number two. With one out in the bottom of the ninth, Jason Bay (who hit .412 in the series) hit a ground-rule double into the right-field seats. After the second out, rookie Jed Lowrie slapped a single between first and second. Bay was running on the pitch and scored easily, setting off the type of wild celebration that Red Sox fans have become used to over the last five seasons.

“What a huge momentum swing for us," said Dustin Pedroia. “We’re just happy to get out of that inning with Tek making a great play, and then in the bottom of the ninth we had some great at-bats and get the run. Unbelievable.”

The Red Sox, who’ve won nine of their last 10 playoff games, are moving on largely thanks to the left arm of Lester. The 24-year-old held the Angels down twice in the series and did not allow an earned run. He’s now pitched 22 2/3 post-season innings in a row without an earned run stretching back to last October. His ERA in three career playoff starts is a miniscule 0.77.

The sight of Lester twirling a gem at Fenway is nothing new for Red Sox fans. He left after seven innings last night with a 2-0 lead, allowing just four hits and two walks. He struck out four, retired the last seven Angels he faced and looked like he could’ve cruised into the eighth even after 109 pitches. Instead, Francona chose to hand the two-run lead to his bullpen. That group quickly lost it.

Hideki Okajima did a fine job by retiring the first two hitters on groundouts. He then carefully pitched to Mark Teixeira and issued a walk. Instead of going to Jonathan Papelbon for a four-out save (he wasn’t even warming up), Francona summoned rookie Justin Masterson to face the dangerous Vladimir Guerrero.

Masterson struck out Guerrero in a big spot in Game Three and quickly got ahead in the count with two strikes. But Guerrero hung tough and worked the rookie for a walk to put two runners on. That brought up Torii Hunter.

Again Masterson jumped out to a two-strike count. On the third pitch, he fired a 97-mph heater that seemed to confuse Varitek as it sailed over his mitt for a passed ball. Teixeira and Guerrero moved up on the miscue. On a 2-2 pitch, Hunter stayed with an outside fastball and stung it into right field for a deflating two-run single that tied the game, 2-2.

The Angels were not done knocking the Sox’ pen around. In the ninth, Masterson gave up a leadoff double off the wall to pinch hitter Kendri Morales. Willits came in to pinch-run and Howie Kendrick bunted him to third. Francona then summoned Delcarmen and the Angels tried to bunt for the win.

“Erick’s a terrific bunter. He feels obviously badly he didn’t get it down," said L.A. manager Mike Scioscia. “It was a buntable ball. Erick just didn’t get it done and that happens.”

In the midst of the champagne, beer and cigars in the Red Sox clubhouse, a group of relieved players weren’t ready to look forward to a trip to Tampa. Bay, a playoff rookie, was counting his blessings.

“The only thing I’m thinking is to get a good jump and don’t fall down around third," said Bay. “I can’t imagine that it’ll be any more intense than this, but it will."

kmcnamar@projo.com