Designated runner Jack Lopez scored on Tom Murphy's passed ball to start the scoring in the six-run 10th to break open a 3-3 game. J.D. Martinez, Schwarber and Vazquez added run-scoring hits as Boston beat the Mariners for the second day in a row.
All three games in the series between playoff contenders were decided in the eighth inning or later.
''It was playoff baseball,'' Red Sox manager Alex Cora said. ''It feels that way. It's fun, to be honest with you.''
The Red Sox, Toronto and the New York Yankees began the day in a virtual tie for the two AL wild-card spots. Seattle started the day three games behind the trio.
Boston gets Thursday off before returning home for eight games against Baltimore, the Mets and Yankees.
''Not too many people thought that the last homestand of the season was going to mean something,'' Cora said. ''Now it means a lot.''
Alex Verdugo singled against Eric Swanson (0-3) to lead off the 10th, moving Lopez to third. Swanson threw a chest-high fastball on his first pitch to Hunter Renfroe, but the ball skipped off the top of Murphy's glove to the backstop to make it 4-3.
One out later, Martinez made it 5-3 with an RBI single.
Adam Ottavino (7-3) pitched 1 1/3 innings for the Red Sox to finish the eighth and ninth innings, earning his second victory in two days. He struck out two.
The score was tied 3-all after three innings.
Boston's Bobby Dalbec reached third in the eighth after a walk, a stolen base and Jose Iglesias' single against reliever Casey Sadler. But Paul Sewald entered with two outs and retired pinch-hitter Verdugo on a popup, preserving Sadler's AL-leading 19 2/3-inning scoreless streak.
And Jarred Kelenic nearly ended it in Seattle's favor in the ninth with a liner to right field that bounced a foot wide.
''In a game like that, you need a few breaks,'' Mariners manager Scott Servais said. ''You need a bounce or a ball to go fair instead of just barely foul.''
''It doesn't look bright right now, but we aren't eliminated and we'll still compete,'' he said.
Renfroe homered in the first and Kevin Plawecki and Iglesias drove in a pair of runs in the second for a quick 3-0 lead against Mariners starter Marco Gonzales.
Gonzales retired 12 of 13 batters after the second inning, striking out seven and not allowing a hit over that stretch.
Seattle rallied in the third. Hits by Kelenic and Murphy, a throwing error by Renfroe from right field and Kyle Seager's two-run double tied it.
Ottavino said the Sox are excited to go home after splitting a road trip with Seattle and the Chicago White Sox.
''I think that gave us momentum,'' Ottavino said. ''We're going to have three series at home, a place we expect to win.''
CORA AND CLEMENTE
Cora proudly wore Roberto Clemente's No. 21 on Wednesday and fondly remembered the stories his father would tell him and his brother, Joey, of the Hall of Fame member when they were kids. Cora said his father, Jose Manuel Cora, played winter ball in Puerto Rico with the former Pirates great, who died in a plane crash Dec. 31, 1972, while delivering aid to earthquake victims in Nicaragua at 38. Those stories have stuck with Cora.
''We don't do enough back home to actually teach our kids who this man was,'' Cora said. ''And I think little by little with the movement of Retire 21 and having this day, little by little, we're getting better at that. But, I mean, with who he was and what he represents, we will always be short, right? Because just his personality, he means so much to us. But I think we're doing a better job now. And I think we're sending a message and the kids are learning who Roberto Clemente was.''
Red Sox: Matt Barnes, Jarren Duran and Christian Arroyo were all scheduled to make rehab appearances Wednesday and could rejoin the club from the COVID-19 list as soon as Friday if all goes well. Barnes was scheduled to pitch at Double-A Portland, while Duran and Arroyo were expected to have at-bats at Triple-A Worcester. Cora hoped he'd have both Chris Sale and Barnes available Friday, but Sale's return has not yet been confirmed.
Red Sox: The team has not announced a starting pitcher for Friday's game against Baltimore in Boston. The hope is Sale will make the start.
Mariners: RHP Chris Flexen (11-6, 3.73) takes the mound in the series opener at Kansas City. Seattle is 18-9 in Flexen's 27 starts.
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