Even Kevin Brown couldn’t sabotage one of the proudest days in Yankee history. Even more unbelievably, Jason Giambi capped it.

The undependable Brown left in the fifth inning of last night’s game with back spasms down two runs, but the embattled Giambi smashed a two-run, upper-deck homer off Jose Mesa to complete a 7-5, 10thinning comeback victory over Pittsburgh.

“It was incredible,” Giambi said. “I’ve been working my [butt] off. Tonight, for one swing, I put it all together.”

The walk-off blast with one out in the 10th came hours after the official unveiling of a sparkling new ballpark to replace Yankee Stadium. It was the Yanks’ first victory when trailing after eight innings this season (they were 0-30).

With a run in the seventh, two in the eighth and the game-tying tally in the ninth, the Bombers segued into Giambi’s extra-inning heroics after being one strike away from a loss.

Before that, they survived a potential game-ending double play when Gary Sheffield was ruled safe at first on a blown call by first-base ump Tony Randazzo.

“We weren’t going to be denied tonight,” manager Joe Torre said. “Guys kept fighting and fighting.”

Countered Pittsburgh manager Lloyd McClendon: “It’s not easy to take, that’s for sure. We’ve got a nice little team here, but we’re not good enough to beat the Yankees twice in their own ballpark.”

The Bombers (32-32) gargled out Brown’s injury-shortened outing and remained in a third-place tie with Toronto, still 6 ½ games behind Baltimore.

In the ninth, Bernie Williams worked a one-out walk off Mesa. Sheffield then appeared to hit into the game-ending double play on a bullet that ricocheted off Mesa to shortstop Jack Wilson, who flipped to second baseman Jose Castillo for the second out of the inning.

Castillo’s throw to first appeared to beat Sheffield by a hair, but he was ruled safe.

“I don’t need to see a replay,” McClendon said. “He was clearly out.” Mesa got ahead 1-and-2 on Alex Rodriguez, who stroked a broken-bat single to center. Jorge Posada doubled off the right-field fence to plate Sheffield from third, but A-Rod ran through third-base coach Luis Sojo’s stop sign and was a dead duck at the plate.

With one out in the 10th, Giambi tattooed a 2-2 Mesa pitch deep into the top deck to plate pinch-runner Russ Johnson after Tino Martinez had drawn a leadoff walk.

Derek Jeter, who embraced Giambi at the plate, actually told him he was going to win the game before the at-bat.

“You’d hope the fans would start cheering him, because if you’re a Yankee fan and you want us to win, we need Jason to do well,” Jeter said.

Giambi admitted being affected by the constant boos.

“I’m a human being,” Giambi said. “It’s not a [stinking] joy – especially in my own ballpark. I’m right there with them, wanting to get that hit.”

Brown’s back tightened when he slipped on the mound sometime in the third, and he gave back a one-run lead. The 40-year-old Yankee righty will be evaluated today, but he’s had back maladies for years.

“Anytime you’ve had back surgery and back problems, it’s impossible not to be anxious,” Brown said.

Brown, who struck out six but allowed six of his seven hits in the fourth and fifth in 4 1/3 innings, was pitching for the first time since June 5 because of a strained left shoulder.

“Looking back, it almost cost us the game,” Brown said of his performance.

Because of Giambi and that old Yankee Stadium magic, it didn’t.

Additional reporting by Evan Grossman