SEATTLE — Normally one to remain stoic and keep his emotions in check, Seattle’s Mitch Haniger couldn’t stop from pumping his fist and screaming as he rounded first base.

He knew the importance of what his 33rd home run meant at this time of year.

“Come September you hope that you’re right in the fight to win the division or make the wild card and it’s been really fun showing up every day,” Haniger said. “There’s a lot of energy and it’s easy to come to the ballpark, and play and work when when you’re still in it.”

Haniger made Boston pay for a costly error by first baseman Kyle Schwarber, hitting a three-run home run with two out in the seventh inning, and the Mariners held on for a 5-4 win over the Red Sox on Monday night.

The opener of a critical three-game series between the AL wild-card contenders went the way of the home team, thanks, in part, to four unearned runs allowed by the Red Sox.

None was more damaging than Schwarber’s mishandling of Jake Bauers’ grounder with two out in the seventh. Reliever Ryan Brasier (0-1) got the ground ball needed to get out of the inning, but Schwarber misplayed the bounce and Bauers was safe.

“It was in the glove. It squirted out and that was it,” Schwarber said. “Frustrated. I wanted to make that play. We had a one, two, three inning and it turns into a lot more.”

J.P. Crawford followed with a broken-bat single and Brasier left a slider in the middle of the plate to Hangier. He didn’t get all of it, but it was enough to clear the wall in left field. Haniger went 4 for 4 and homered for the third straight game.

The Mariners also scored an unearned run in the second after an error by right fielder Hunter Renfroe. The Red Sox have allowed 33 unearned runs since the All-Star break, most in the American League.

“It’s frustrating, but we just got to keep working at it,” Red Sox manager Alex Cora said. “We’ve got to put in the work and hopefully get into a ‘hot streak’ defensively that we play clean baseball. When we do that we’re really good.”

Boston didn’t go quietly. Xander Bogaerts and Rafael Devers hit back-to-back homers with two outs in the eighth off Seattle’s Paul Sewald. It was the third straight appearance Sewald has allowed a long ball after allowing just four in his first 48 games this season.

Sewald escaped the inning, and Drew Steckenrider pitched the ninth for his ninth save. Seattle pulled within two games of Boston in the jammed AL wild-card standings.

STARTING OFF

Seattle rookie Logan Gilbert threw six strong innings in his best outing since early July, when he threw seven shutout innings against the Yankees. Rediscovering the feel for his slider, Gilbert allowed five hits and matched his career high with nine strikeouts. The most important of those strikeouts came against the final batter he faced. With Bogaerts at third following a leadoff double, Gilbert struck out Alex Verdugo to end the sixth inning and keep the game tied.

“He looked every bit of a big game pitcher for a guy who’s in his rookie season. It’s pretty fun to see,” Seattle manager Scott Servais said.

Jose Iglesias homered for the first time since Aug. 6, and later had a bloop RBI single off Gilbert.

Boston starter Eduardo Rodriguez allowed six hits and one earned run over six innings. He struck out six.

TRAINER’S ROOM

Red Sox: Boston activated reliever Hirokazu Sawamura from the COVID-19 injured list and returned left-handed pitcher Austin Davis from the paternity list. Cora said there could be more players coming off the COVID-19 injured list in the coming days. Left-hander Martin Pérez is expected to be activated on Tuesday, and Cora is holding out hope that left-handed starter Chris Sale could return this weekend against Baltimore.

Mariners: OF Jake Fraley (shoulder) could be activated from the injured list in the next couple of days following a rehab stint with Triple-A Tacoma, Servais said. Fraley has been out since Aug. 28. … RHP Justin Dunn (shoulder) threw a second simulated game on Monday and could head out to Tacoma or Single-A Everett for a rehab stint, depending on how he feels. Dunn has been on the injured list since late June.

UP NEXT

Red Sox: RHP Nathan Eovaldi (10-8, 3.57) threw seven scoreless innings in his last start against Tampa Bay. Eovaldi is 1-1 with a 1.91 ERA in his past six starts with 47 strikeouts and six walks during that span.

Mariners: LHP Tyler Anderson (6-9, 4.20) makes his ninth start since being acquired by Seattle. Anderson is 1-1 with a 3.83 ERA in his time with the Mariners, but lasted just 4 2/3 innings in his last start against Houston.