SEATTLE -- The pendulum swung swiftly in the span of mere minutes on Wednesday afternoon at T-Mobile Park, where the Mariners went from on the cusp of a would-be walk-off to a disappointing defeat at the hands of the very team ahead of them in their hunt for October.

Twelve pitches after Jarred Kelenic sliced a line drive into the right-field corner but foul, eventually leading to a strikeout, Boston took a decisive lead on a passed ball from Erik Swanson to Tom Murphy one half-inning later in the 10th, sending Seattle to a 9-4 loss.

“I was hoping,” Kelenic said of his thoughts off the bat. “I got a good pitch to do something with and put a good swing on it, hit it real hard and, you know, it is what it is.”

With the defeat, the Mariners (78-68) fell to four games behind the Red Sox, who are tied with the Blue Jays and Yankees for the top American League Wild Card spot.

Swanson, who faced the challenge of entering with a lead runner -- which wound up being the go-ahead run -- couldn't make it out of the jam, leaving with the bases loaded for Justus Sheffield, who induced a forceout to Rafael Devers but then gave up consecutive singles to J.D. Martinez and Kyle Schwarber. Yohan Ramirez then allowed those two runners to score on a double by Christian Vázquez that blew the game open.

“I have nothing but praise and how we approached things today and how we got after it again,” Mariners manager Scott Servais said. “Our bullpen guys at the end did a heck of a job there to keep it tied. We just ran out of juice there in the 10th inning.”

As was the case in a stinging loss Tuesday, Wednesday’s box score was more lopsided than what transpired before the decisive 10th.

Kelenic had the 17,860 on hand for the midweek matinee roaring on their feet in a 3-3 game with a tangible feel of fall in the air on this 59-degree day -- and an intangible feel of October given the weight of the stakes. But the margin between what would’ve been the biggest moment of the rookie center fielder’s career and an 0-2 count proved to be inches.

On a day in which he scored the Mariners’ first run, had two hits and a stolen base, Kelenic yanked an inner-half, 95.7 mph fastball from Adam Ottavino just barely outside the playing field while Jake Bauers was rounding second base in a hit-and-run attempt -- one that, had the ball landed fair, would’ve allowed him to score easily.

Instead, Kelenic returned to the batter’s box against one of the game’s tougher righties, worked the count full in an impressive battle, but ultimately struck out swinging on top and out front of an 88.4 mph changeup to end the inning.

“You just go off of the homework that you have,” Kelenic said. “We were looking at the scouting report, and when he was ahead in the count, he threw a lot of sliders. So I was looking for something out over the plate, and really, my hands are quick enough to react to the fastball and just try to fight that pitch off. I was kind of looking for that hanger that he was going to try to freeze me on, and I think he saw me foul off a fastball there and brought me to the next pitch. And then, he throws his changeup like 3 percent of the time, and that 3 percent of the time, he used it against me there.”

According to Statcast, Ottavino has thrown his changeup just 1 percent of the time -- and exclusively to left-handed hitters.

Bauers represented the Mariners’ fifth runner stranded on Wednesday, bringing their total to 35 on this disappointing, 2-4 homestand, over which they collectively hit .211/.271/.377 (.648 OPS). After Bauers stole second following the foul, Kelenic’s decisive at-bat was part of a 1-for-9 showing with runners in scoring position, bringing their total to 8-for-39 these past six games.

“A game like that, you need to need a few breaks,” Servais said. “You need a bounce or ball to go fair when it's just barely foul, things like that. And obviously, we did not get that to happen for us at the end.”

The 10th inning spoiled what had been a stellar collective pitching effort from Seattle’s bullpen, one day after it surrendered seven runs, and Marco Gonzales, who threw six strong innings in what continues to be a huge second-half turnaround.

Seattle now embarks on a three-city, 10-game gauntlet of a road trip to Kansas City, Oakland and Anaheim that will likely determine its fate. The Mariners have been so resilient in flushing away tough losses all season, but they now face the challenge of needing help. There are three teams ahead of them in the playoff hunt with just 16 games remaining.

The Mariners still have seven games against the A’s, but Wednesday represented one of their final opportunities to do direct damage on a club they’re chasing. Their uphill climb continues -- and with time running out.