SEATTLE -- Run prevention proved to be the Mariners’ biggest pitfall on a night when they had their division leader on the ropes, in their building and relying on a shaky bullpen while playing behind.

Despite a strong chance to start this series with Houston with a victory, two defensive miscues on balls hit to left fielder Dylan Moore proved to be the costly difference in a fateful eighth inning that saddled Seattle with a 4-3 defeat on Monday at T-Mobile Park, its fourth in five games on this seven-game homestand.

Moore, who had just crushed a go-ahead, pinch-hit, two-out homer in the sixth inning, and J.P. Crawford seemingly miscommunicated on a leadoff single by Yuli Gurriel that led to a passed throw to the Gold Glove shortstop, who was charged with an error, allowing Gurriel to reach second base. He scored in the ensuing at-bat, when Kyle Tucker pulled a single to right field that would’ve otherwise held Gurriel at third. 

Two batters later, Tucker scored from second when Moore hesitated after fielding a single in left-center field from Jake Meyers, instead holding the rookie outfielder at first base. Tucker was rounding third and appeared to slip in the process as Moore was fielding the ball, which made Moore’s hesitation all the more costly.

“It was the wrong read, and I should have thrown home,” Moore said. “That's the go-ahead run, the deciding run. I’ve got to throw home. I can't really pinpoint a reason other than the ball took a while to get to the outfield. Yeah, [Tucker] did stumble, and then I just made the wrong read and didn't throw it when I should have.”

There was also a top-spinning line drive from Yordan Alvarez in the first inning that turned Jarred Kelenic around at the center-field wall and led to a bobbled catch attempt, which allowed Michael Brantley to score the game’s first run from second base. It also advanced Carlos Correa to third, who then scored on an aggressive and equally impressive slide around a tag from Tom Murphy while tagging up on a sacrifice fly to Kelenic in shallow center.

Those moments loomed large for a Mariners team that has now played in 43 one-run games, more than any team but the Mets, who’ve played in 50. With Monday night's loss, Seattle is 27-16 in such contests.

“We just didn't execute defensively as [well] as we normally do,” Mariners manager Scott Servais said, “ ... whether it's catching the ball coming in, throwing to the right base, communicating correctly. It's something we've been very, very good at throughout the course of the year. And tonight, we did make a couple mistakes there that hurt us, and if you do that against a good team, they will take advantage of it.”

Despite a leadoff walk by Luis Torrens in the bottom of the ninth, Seattle went quietly, striking out four times in the final two innings. Perhaps the most frustrating element within the Mariners’ clubhouse is that each of the defeats on this critical homestand were winnable.

To be sure, a one-run lead at the time of Moore’s miscues and three runs total seemed like too thin a tightrope to walk against the American League’s best offense -- even on a night where Chris Flexen overcame shaky command early to finish 5 2/3 innings with just two runs allowed.

The Mariners, who stranded 37 runners in last weekend’s four-game series against Kansas City, left seven on the bases on Monday, including three innings with runners on the corners. For a team whose .812 OPS with runners in scoring position ranks second in MLB to only these Astros, leaving that many on in these moments has been a critical component to the club's inability to reach the finish line victorious this past week.

“It's harder to hit with runners in scoring position,” Servais said. “Pitchers are focused. They're locked in. They have been making pitches against us, and we just haven't had productive at-bats over the course of the homestand. It's frustrating. You get traffic out there, you feel like you got a big inning starting to come together for you, and you’ve just got to keep the line moving. And we haven't been able to do it.”

With the loss, the Mariners fell to 4 1/2 games back of Boston for the second AL Wild Card, where they are tied with Toronto and looking up at Oakland (which is two games back). It’s a crowded field with just 30 games to play, though the race is hardly over. But getting beaten at their own game -- and beating themselves -- is far from the formula that they want to take into the season’s final month.