BOSTON — New half, same leaky bullpen.

After a terrific job by three relievers, Aroldis Chapman walked home the winning run in the bottom of the ninth inning Friday night as the Red Sox handed the Yankees another in a series of crushing one-run losses, this one 5-4 in front of 37,570 at Fenway Park.

It was the 18th blown save of the season for the Yankees (45-42), who have lost 19 of their last 26 games, and the 10th blown save in their last 13 chances. The Yankees fell to 9-18 in one-run games and are 2-13 in their last 15.

“It’s frustrating because we’ve put ourselves in situations to win some games and we haven’t been able to close them out, for whatever reason,” Joe Girardi said. “But you have to win those games. If you want to win championships, you have to win those games.”

Boston did not hit the ball out of the infield in the ninth but scored twice against Chapman, who was unable to retire any of the five batters he faced (Ronald Torreyes’ error contributed to that).

The Red Sox (51-39), who earlier in the day designated third-base bust Pablo Sandoval for assignment, saw their lead over the Yankees increase to 4½ games.

Chapman allowed infield singles by Mookie Betts and Dustin Pedroia to start the ninth, and a double steal put both runners in scoring position. Xander Bogaerts then scorched one to second that Torreyes booted for his second error of the night. That allowed Betts to score the tying run and moved Pedroia to third.

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“It’s one of those plays where the game’s on the line,” Torreyes said through his translator. “If you make the play, we probably have a better outcome.”

After an intentional walk to Hanley Ramirez — who had hit a two-run homer earlier in the night — loaded the bases, Andrew Benintendi walked on five pitches to end it.

Though Chapman still regularly reaches 100 mph with his fastball — the lefthander reached 102 mph Friday night — he has had difficulty finishing off batters, a trait that had ranked him among the most feared closers in the game.

He did not give a reassuring answer when asked why that has been the case of late.

“That’s a good question,” he said through his translator. “Honestly, I don’t know why.”

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Said Girardi: “As I’ve said, guys are getting a little more used to seeing hard throwers. It’s important that he puts guys away quickly and uses all his pitches.”

Entering the ninth, it had been a highlight night for the bullpen, called on early as rookie lefthander Jordan Montgomery saw his pitch count quickly balloon. He threw 96 pitches in his four innings, allowing three runs, six hits and three walks.

An electric Chad Green struck out the final five batters he faced in two perfect innings, the fifth and sixth, and Adam Warren stranded two in the seventh. He was fortunate that Benintendi launched a fly ball to the deepest part of Fenway, the triangle in center. Dellin Betances, who had a 14.29 ERA in his previous eight appearances, struck out three in the eighth.

The finish overshadowed a good night at the plate by Gary Sanchez, who went 2-for-4 with three RBIs after a 2-for-20 skid going into the break.

Sanchez’s monstrous two-run homer, which landed on Lansdowne Street beyond the Green Monster seats in leftfield, highlighted a three-run fifth that gave the Yankees a 4-3 lead. It was Sanchez’s 14th homer.

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Until the last inning, it seemed as if the Yankees would kick off this three-city, 11-game trip in a positive fashion. Instead, there was even more frustration.

“It would have been really nice to win, but I don’t think you can make too much out of one game,” Girardi said. “I really believe I’ll wake up tomorrow and, I don’t know if it will be sunny, but the sun will come up.”