BOSTON _ What was once an early Yankees lead had disintegrated at Fenway Park, during a dizzying, eight-run fourth inning by the Red Sox.
And all through the night, Boston reminded their visiting rivals about which club was currently in command of the AL East _ with homers, soft hits, big defense and baserunning that made the Yanks look amateurish.
Boston's vivid display of dominance culminated in a 15-7 win, dropping the Yankees to 6.5 games behind.
Steve Pearce clubbed three home runs for the Red Sox, off three separate Yankees pitchers _ beginning with starter CC Sabathia.
The Yankees' reliable veteran lefty lasted just three innings, charged with two runs.
Entering the game, Sabathia (6-5) had been 12-0 with a 2.72 ERA in 20 starts following a Yankees loss since the start of the 2017 season.
Wednesday's 7-5 loss to the lowly Orioles might have been the Yankees' low point of the season, leading to Sonny Gray's demotion to the bullpen and a dugout scolding by third base coach Phil Nevin about the club's lax defense.
Thursday's effort sunk the Yankees (68-39) even lower, with reliever Jonathan Holder tagged for seven runs _ forcing manager Aaron Boone to use Luis Cessa in long relief.
Cessa had been penciled in to start Saturday in place of J.A. Happ, who wound up on the 10-day disabled list with Hand, Foot and Mouth Disease.
As 37,317 fans began heading for the exits, it was hard to recall that this was the latest in-season matchup between the Red Sox (76-34) and Yankees with the game's two best records since Oct. 2, 1978.
That was the "Bucky Dent Game," when the Yankees beat the Red Sox at Fenway in a one-game playoff for the division title.
Things seemed to break right for the Yankees, even before Thursday's first pitch.
They didn't have to face Boston's lefty ace Chris Sale, sidelined for at least one start due to left shoulder inflammation.
And Boone's club jumped lefty starter Brian Johnson right away.
A three-run homer by Didi Gregorius _ the first of two on Thursday _ in the first inning and a solo shot by Aaron Hicks in the second had provided Sabathia with a 4-0 lead, which was whittled down to 4-2 entering the fourth.
Sabathia walked Mookie Betts with the bases loaded in the second and watched Andrew Benintendi fly out to the left field warning track to end the inning.
In the third, Sabathia absorbed a hard, Blake Swihart comebacker off his left hip area, but he still fielded the ball _ only to throw it into right field. Boston didn't score in that inning, but Sabathia was done after 77 pitches.
On to start the fourth, Jonathan Holder couldn't retire any of the seven batters he faced, and was ultimately charged with seven runs.
Before Pearce's three-run homer off Holder, the right-hander might've been rattled by a goofy play where he fielded Benintendi's sharp comebacker _ only to be deked by Jackie Bradley Jr. at third base.
Bradley Jr. wound up beating Holder's late throw to the plate, and the Red Sox were off to the races. Chad Green was finally called in, and another subpar outing forced Cessa to finish the fourth.
Cessa wound up going 3 2/3 innings, yielding five earned runs.
And Pearce, who played 12 games for the Yankees in 2012, went from five homers to eight on the year, while driving in six runs.
Holder's ERA went from 2.06 to 3.50 in one night.
Before the Red Sox evened the season series (5-5) against the Yanks, with nine to play, Boone saw the opportunity to claw back during this four-game series.
"Obviously, they're a great team having an amazing year. We know the challenge, especially in this place, against them.
"You want to play against the best," Boone said. "We do look at this as a great opportunity this weekend to hopefully get some things done."
The best they could accomplish on Thursday was Gregorius's fourth multi-homer game of the year and a monstrous shot by Giancarlo Stanton, well over the Green Monster, during the blowout portion.
Johnson (2-3) gave up four earned runs and three homers in five innings, but struck out 11 Yankees.
