With one deep fly ball of the bat of Jarrod Saltalamacchia, the Yankees went from inching closer to the Rangers and Rays in the wild card race to being buried.
Saltalamacchia's eighth-inning grand slam was the hammer punch in the Red Sox's 8-4 win, pushing their record to 89-50.
For the Sox, who have won eight of their past 10, it's their fifth 90-win season since 2007 but their first since 2011.
After snapping an 0-for-21 slump Thursday night against Tampa Bay, Saltalamacchia went 2 for 3 with two runs scored, and four RBIs in the opener of a three-game set against New York.
Despite going 6.1 innings and giving up four earned runs, Sox starter John Lackey didn't factor into the decision.
End of the seventh, Red Sox 8, Yankees 4: When John Lackey gave up back-to-back one-out singles to Brendan Ryan and Chris Stewart, Sox manager John Farrell immediately went to the bullpen.
He had matchups in mind. With Curtis Granderson and Robinson Cano sandwiching Alex Rodrguez in the lineup, he went to lefthander Craig Breslow.
The numbers were on his side. In his last 25 outings, Breslow was 3-0 with a 0.70 ERA, but beyond that, Granderson was 2 for 14 against Breslow all-time and Cano was 4 for 18.
The wild card was Rodriguez, who was 2 for 6 against Breslow but 0 for 3 on the night.
At first, everything went according to plan. Breslow won a seven-pitch battle with Granderson, who tried desperately to check his swing but couldn't keep himself from chasing a slider down and away (his protest to home plate umpire Ted Barrett was futile).
But things fell apart when he pitched around Rodriguez to load the bases.
That brought up Cano, who sent a line drive screaming to the gap in right-center for a game-tying two run double.
But when Yankees manager reached into his bullpen in the bottom of the inning, the Sox were ready.
Leading off the inning, Shane Victorino shot a liner to third with so much steam that all Yankees third baseman Eduardo Nunez could do was stick his glove out and get his body out of dodge for self-preservation.
The hit was huge. After David Ortiz took a fastball off the arm (one of just two pitches by Yankees reliever Cesar Cabral), Jonny Gomes worked a pinch-hit walk to load the bases.
Daniel Nava struck out swinging, but Jarrod Saltalamacchia was able to cash in big.
His launched a deep fly ball over the Red Sox's bullpen for a grand slam that put the Sox back up by 4.
The shot may have very well buried the Yankees, who had a shot at gaining ground in the wild card race with Oakland pounding the Rangers 8-2.
End of the sixth, Red Sox 4, Yankees 2: The Yankees managed to squeeze a run out of Robinson Cano's one-out double, Alfonso Soriano's single and Lyle Overbay's sacrifice fly to right.
But even with Soriano swiping his 18th base to get into scoring position with two outs for Eduardo Nunez, they couldn't push another run across to make it a one-run game.
The Yankees have a weird way of sticking in games. They've won 11 times this season when they've trailed after six. Their past two wins have both come with the score tied through 8.
If you're looking for another magic (or arbitrary) number, try four. The Yankees are 66-27 when their opponent scores four runs or less. They're 12-41 when they give up 5 or more.
Meanwhile, Craig Breslow is up in the bullpen with John LAckey at just 75 pitches.
End of the fifth, Red Sox 4 Yankees 1: Somehow Hiroki Kuroda's still hanging on.
It looked like his night was about to be done when Yankees pitching coach Larry Rothschild made a mound visit in the third, but since giving up four runs in the first, he's managed to stay trouble free, retiring nine of the past 11 batter's he's faced.
Through five innings, he's at 87 pitches and a night that could have been disastrous somehow seems salvageable.
End of the fourth, Red Sox 4, Yankees 1: It's rare that Dustin Pedroia commits an error. He only had four coming into tonight. But he had another one hung on him on a one-hopper that was weirdly similar to another one that came his way almost a month ago. But that one was ruled a hit.
They were both against the Yankees. Both came at Fenway. Both were a tricky liners that blew up on him late.
This one was off Lyle Overbay's bat. It was hit hard to his glove side, he reacted by sticking his glove out, but not getting in front of it. It bounced off his leather and into right field.
He was, predictably, cursing mad.
The other was off the bat of Alex Rodriguez. It was hit right at Pedroia, low with topspin. Pedroia tried to block it with his body, but it kicked off his glove.
Even though he got the benefit of the doubt from the scorer though, who ruled it a hit, Pedroia was just as disappointed with himself for not making the play.
Pedroia's five errors this season match his total for 2012.
Of course, he ended the inning by coming up with a diving stop on an ground ball by Ichiro in the hole with runners at the corners, saving a run.
End of the third, Red Sox 4, Yankees 1: John Lackey is relentlessly -- and almost exclusively -- firing fastballs at the Yankees lineup.
But Brendan Ryan was able to get a hold of one.
He got a head of Lackey 2-and-0 and then he waited. When Lackey left and belt-high fastball over the plate, he smashed it over into the monster seats, putting the Yankees on the board.
But like most of the 22 homers, Lackey's given up this season, it did little damage.
It was the 18th bases-empty homer he's allowed this season.
End of the second, Red Sox 4, Yankees 0: Two things stick out about John Lackey's first two innings.
First, he's gotten threw them in just 22 pitches.
The Yankees made it easy on him in the second. Lyle Overbay, Eduardo Nunez and Ichiro Siuzuki went down 1-2-3 on seven -- yes, seven -- pitches.
Second, 17 of those have been fastballs. His heater's averaging 91.6 miles per hour. He's used it twice to punch out Alex Rodriguez and Alfonso Soriano swinging.
After going giving up seven runs in 5.2 innings to the Yankees in his last start, he's in command so far tonight.
End of the first, Red Sox 4, Yankees 0: When Dustin Pedroia came to Red Sox manager John Farrell and suggested that he move into the leadoff spot in Jacoby Ellsbury's absence, Farrell didn't make it sound like he questioned it at all.
The positives were obvious, Farrell figured.
Pedroia sees 4.09 pitches per plate appearance, which meant that Shane Victorino would could get a pretty good idea what pitchers were throwing hitting behind Pedroia in the two-hole.
But more than that, it meant David Ortiz could move into the three-hole and get a guaranteed at-bat in the first inning.
The plan worked out step by step in the first.
Pedroia roped an 0-and-2 pitch to left to start thing off (he's now hitting .357 as the leadoff man), and after Victorino flew out to center, David Ortiz ripped a double into the right-field corner.
Pedroia scored on Mike Carp's ground ball to first to give the Sox the early lead, and they added to it when Daniel Nava smacked a single to left to plate Ortiz.
With Yankees starter Hiroki Kuroda laboring (he threw 33 pitches in the inning), the Red Sox gladly made things even harder for him.
Kuroda walked Jarrod Saltalamacchia on four pitches, then Stephen Drew put together a six-pitch at-bat that ended with him shooting a fly ball deep into the right-field corner for a two-run double.
Through just one inning, Kuroda's line allready reads like a night's worth of work: four runs on four hits with a walk.
Pregame: It's hard to explain exactly how the Yankees -- battered by injury, embattled by scandal and largely written off at the start of the season -- are still a living breathing playoff contender, but they are.
Coming into this three-game set at Fenway, they sit a game out of the wild card.
And this is the time of year when they typically heat up.
Since 2008, the Yankees are 93-65 in the seasons' final month which,the third-best mark in the majors. Since 2001, they baseball's best regular-season Sept./Oct. record, 223-132.
Before the game, Sox manager John Farrell admitted the race for the wild card is interesting.
"I can't imagine what the final week of the season is going to be," he said.
But with a magic number of eight, the Sox have other things to worry about.
"I'm not even considering who we see," Farrell said. "We've got to get there first and that's the beacon."