OAKLAND — No team in the American League wins at a more regular rate behind a single starting pitcher than the Mariners have behind Chris Flexen.

Even Seattle’s front office couldn’t have seen that coming when it brought in the late-blooming right-hander on a team-friendly contract last December to help fortify what they intended to be a six-man rotation.

Yet here Flexen is, adding more credibility to his Major League resurgence, now 29 starts into his breakout year. The Mariners have won 20 of them after Wednesday’s 4-1 victory over Oakland at the Coliseum, good for a .690 winning percentage that is best in the AL and trails only three other pitchers: the Dodgers’ Julio Urías (.800), Max Scherzer (.714), who was dealt to Los Angeles from the Nationals at the Trade Deadline, and the Brewers’ Corbin Burnes (.692). Those last two will likely be National League Cy Young Award finalists.

The Mariners still have 10 games remaining to climb their way into the AL Wild Card Game, but if they do reach that point, there’s a strong chance Flexen would have some serious consideration to start.

“I can say this about a few players in that clubhouse right now — we would not be in this space, in this spot without the efforts and what they’ve given us this year — but maybe none bigger than Flex,” Mariners manager Scott Servais said.

Speaking of the postseason standings, Flexen’s efforts — along with Kyle Seager’s 35th homer and career-high 100th RBI — helped push the Mariners (83-69) to 2 1/2 games back of the final spot currently occupied by the Yankees (86-67) and two games back of the Blue Jays (85-67), who lost to the Rays earlier in the day.

The Mariners, who supplanted Oakland (82-70) and have now won eight straight against the defending AL West champions, need help in the way of losses by the teams they’re chasing.

But they continue to do their part by taking care of business, so to speak, advancing to 5-1 on this 10-game, make-or-break road trip, including wins in both of Flexen’s starts in this stretch.

The Mariners wouldn’t be in this position without this renaissance from Flexen, who doesn’t possess overpowering stuff but finds his edge in perhaps the oddest of ways, at least because of how effective it is: relentlessly harsh self-criticism.

Those moments have practically become memes during Mariners broadcasts, and there was another one Wednesday notable for how excessive it felt. Flexen surrendered his only run on a center-cut fastball that Matt Chapman crushed 414 feet over the wall in straightaway center that led off the bottom of the fifth inning.

Flexen was able to prevent further damage, but three outs later, he unloaded an expletive-filled tirade at himself while walking off the mound.

Yet after all that gnawing of bubble gum and shouting into his glove, he went on to finish his outing with consecutive 1-2-3 innings in the sixth and seventh, including three strikeouts that brought him to a season-high-tying eight.

“Flex is so hard on himself, probably more so than any pitcher I’ve been around in quite some time,” Servais said. “I love his competitiveness. He’s really driven to show people that this isn’t a fluke. He’s real. And I certainly am a believer, and anybody that’s watched him all year should be a believer as well.”

“A lot of guys have helped me along the way, consoled me during the game and told me everything’s all right … and it’s just kind of like a little recheck,” Flexen said of his intense demeanor on the mound. “That’s just who I am. I do think it can get better, [I can] be more efficient and definitely make sure it doesn’t take you out of a game.”

The stories of self-shame in the big leagues are so endless in this mentally-taxing pastime, but rarely are they as prominently on display as Flexen has made them this season. Even more rare is his ability to not let those moments get the best of him.

How does he avoid succumbing to himself?

“It’s gotten better over the years,” Flexen said. “I’ve definitely struggled with that in the past, and I think it’s something that I continue to try to work on as well because it is something that can take you out of the game and to dwell on one pitch and ruin the rest of the game is, it’d be dumb. But yeah, it’s reaction. I get on myself, and then I try to take a deep breath and refocus because I have a job to do.”

Beyond the fortification of and faith in his repertoire, featuring a much-improved fastball, it’s that psychological edge that has made Flexen the most consistent — and reliable — starting pitcher on Seattle’s staff. He’s a huge reason, perhaps the biggest, why they’re on the cusp of the playoffs, and if they do get in, he’ll be a huge factor in how far they go.