Bay's rocket to center field against Mariano Rivera soared just over the Green Monster. His two-run equalizer on a 1-0 cutter electrified Fenway Park and set the stage for a 5-4 victory for the Red Sox in 11 innings.
Kevin Youkilis sent everyone home by hitting a towering shot to left that went over everything and onto Lansdowne Street with one out in the 11th. The blast came against Yankees left-hander Damaso Marte, giving the Red Sox their eighth win in a row.
According to the Elias Sports Bureau, it was the first time the Red Sox have had a game-tying homer against the Yankees in the ninth and then a walk-off long ball in extra innings.
"A lot of unique things happen in this ballpark," said Red Sox manager Terry Francona. "Part of it is that we have good players. I do think the atmosphere leads to that. I remember being a visiting team. There can be some doubt that creeps in. I know it's Mariano, but we always feel like we have a chance here. We've won some pretty good games here."
Just like that, the rivalry kicked into full gear as the first of 18 meetings wound up as a rewarding comeback win by the Red Sox.
"I just go up there and try to get a good pitch to hit," said Youkilis. "Luckily, I got a pitch there at the end, a fastball, and took a swing at it and had a good result."
Youkilis has had a lot of those of late, as he's now hitting .433 with five homers and 13 RBIs.
"He's very good at waiting for his pitch," said Bay. "Obviously, he's very patient out there. Right now, he's just kind of doing everything right. He's not missing."
"There's a lot of good rivalries, but it seems like every time we play the Yankees, something good's happening for either team," said Red Sox second baseman Dustin Pedroia. "We've been on the wrong end of games like this just as much as they have, so it's been crazy."
It was Rivera's 12th career blown save against the Red Sox, the most against any team during his illustrious career.
The way Bay sized up the situation, all the pressure was on Rivera.
"It's a no-lose [situation]," said Bay. "You're going up against one of the best ever. You're probably supposed to make an out, so why not go down fighting and see what happens? More often than not, you're not going to do that, but on a night like tonight, anything can happen."
"I just made a bad pitch, that's all it is," said Rivera. "If I made my pitch, it would be different. I wanted it to hit the wall, but it didn't. That was the game."
Well, not quite. The Yankees had two more innings to climb back on top. And they nearly did against Sox closer Jonathan Papelbon in the 10th.
With Jose Molina on second and one out, Derek Jeter ripped one up the middle that seemed to have hit written all over it. But Pedroia lunged behind the bag for a diving stop and fired to first to get Jeter.
"He hit the ball real hard," said Pedroia. "I was able to get the glove on it and caught it. So it was one of those things where I was kind of in the right spot at the right time."
After a Johnny Damon walk, Mark Teixeira, the Yankees' prized new bat, stepped to the plate with a golden chance to become a hero in his first rivalry encounter. Papelbon fell behind 2-0 but bounced back quickly, striking out Teixeira on 96-mph heat.
The teams -- backed by starters Jon Lester and Joba Chamberlain -- had played to a 2-2 standoff after six.
Immediately after Lester's exit, the Yankees rallied against Hideki Okajima to start the seventh. In fact, Okajima faced four batters and didn't get any of them out. Teixeira put the Yankees in front with a bloop RBI single to center.
After Teixeira's hit fell and Jorge Posada followed with a single to right, Okajima exited with the bases loaded and nobody out. Manny Delcarmen did well to minimize the damage from there, allowing only a sacrifice fly to Robinson Cano that made it a two-run game. That wound up being big.
Rivera had looked like the future Hall of Famer he is while snuffing out a two-on, two-out rally and striking out Pedroia looking on a nasty inside cutter to end the eighth.
It seemed like the Yankees were going to bust it open in the top of the ninth, loading the bases with nobody out against Sox left-hander Javier Lopez. Somehow, though, Boston kept the score where it was. With the infield playing in, Cano hit a crisp grounder to Pedroia, who fired to the plate to spark a 4-2-3 double play.
Perhaps that was the sign that things were finally starting to turn.
For early on, the Red Sox were thoroughly frustrated, hitting into four double plays over the first five innings against Chamberlain.
"Fortunately tonight, we hung on," Francona said. "That could have gone in a lot of different ways. Early on, we had chances and they had chances. We gave ourselves a chance to keep playing."
And, eventually, heroes would emerge.
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.