BOSTON — This is Jon Lester unleashed: unhittable in May, a stopper in October, the emerging ace of the Boston Red Sox.

The 24-year-old lefty will start Game 3 of the AL Championship Series against Tampa Bay today, facing Matt Garza at Fenway Park under the championship banner Lester helped raise with a World Series-clinching victory just months after he was cured of cancer.

"We're one of four teams left right now. You're playing for the ultimate prize. You're playing for another ring, a chance to be world champions again," Lester said yesterday before Boston's off-day workout. "I think a lot of the young guys that were here last year get spoiled. We're doing it again. It's almost like we expect it, and this is all we know."

All Lester has known lately is success — especially at Fenway.

He went 11-1 at home this season, including a May 19 no-hitter against Kansas City. He has not allowed an earned run in his last 22 2/3 postseason innings, including the Game 4 victory in the World Series as the Red Sox swept Colorado to win their second title in four seasons.

"He's pitching unbelievable right now, and I think he's become our ace," Kevin Youkilis said. "Coming here at home he always pitches pretty well, so we've just got to have him pitch a good game and jump on Garza as soon as we can."

The Red Sox took the opener of the best-of-seven series in Florida, but the Rays tied it Saturday night when B.J. Upton lofted a bases-loaded sacrifice fly to shallow right field in the 11th inning to give Tampa Bay a 9-8 victory. Josh Beckett, dominant in Boston's '07 title run, allowed eight runs in 4 1/3 innings, taxing the bullpen and leaving the Red Sox in need of a Game 3 rescue from Lester.

"He's the guy you want to give the ball to," Boston reliever Manny Delcarmen said. "With what he's done, he's the guy everybody feels comfortable in after a tough loss that he's going to go out and throw well."

Lester was diagnosed with anaplastic large cell lymphoma, a rare cancer of the lymph nodes, as a rookie in 2006. He was pronounced cancer-free after chemotherapy in December and cleared to return to the mound in the spring of '07.

Although there's little precedent for a pitcher recovering from non-Hodgkins lymphoma, the Red Sox did what research they could on how to get him back to full strength. Their conclusion: take it slow.

"I made a call to his folks in spring training, and I told them, 'We're really going to (upset) your son.' And they laughed," Red Sox manager Terry Francona said. "They said, 'You know, we understand why.'"I think it was the correct thing to do. He was pitching fine, but it wasn't what it is now. I don't think you ever expect that, but when you're competing you want it to be there — right now."

The Red Sox started Lester in the minors in 2007 before calling him up to Boston in late August. He went 4-0 with a 4.72 ERA in 11 starts over the past two months of the regular season, and when the playoffs began they sent him out to the bullpen.

He did not appear in the first round against the Angels, and he came out of the bullpen in a pair of losses to Cleveland in the ALCS. In the World Series he started the fourth game and pitched 5 2/3 shutout innings against the Rockies.

Lester agrees now that it was smart to take it slowly, but he also admits that it was frustrating to be held back when he wanted to be just another pitcher on the staff.

"You don't ever want to have restrictions when it comes to what you do," he said. "It was frustrating at times, but in the end it was the best thing for me."