OAKLAND -- Even with Frankie Montas establishing himself as ace of the staff for this final stretch of the season, the A’s could really use a second rotation member to emerge as a co-anchor.

Sean Manaea has done it before this season, and he just might be ready to do it again.

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Following a dominant performance by Montas in Wednesday night’s game, Manaea matched his fellow rotation mate with a superb effort in Thursday afternoon’s 3-1 victory over the White Sox at the Coliseum. The left-hander struck out nine over seven innings of one-run ball, leading the way for Oakland to secure an important series victory over Chicago.

The win also pushed the A’s closer to the second American League Wild Card spot at two games back of the Yankees, who lost to the Blue Jays on Thursday night.

“This was a big series for us,” A’s manager Bob Melvin said. “Coming off a really tough one in Toronto and losing the first game in this series, we know we have to win a lot of series coming down the stretch. This looked to be a really difficult one.

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“To come back and win two games shows you what our team is made of. I think everybody here expects to get on a roll. Hopefully, these two wins catapult us.”

After enduring a career-worst month of August in which he posted a 9.90 ERA over five starts, Manaea appears to be rounding into form, having completed seven strong innings in back-to-back outings. Given his history of success over the final month of the season, the A’s feel good about Manaea being able to keep this up. Through 19 career September starts, Manaea is now 11-3 with a 2.14 ERA.

“He stepped up,” Melvin said of Manaea. “This time of year, you tend to be a little tired. His last two starts have been anything but. Coming down the stretch is when you expect big things from guys like him. Last two times out, he’s been as good as he’s been all year.”

This version of Manaea bears a resemblance to the one many considered to be an All-Star snub after the impressive first half he turned in. Back then, he relied on good command of his sinking fastball to fill the zone with strikes, enabling him to routinely pitch deeper into games.

On Thursday, Manaea, who entered the day with the highest sinker usage of any starting pitcher in the Majors at 59.2 percent of his total pitch output, utilized the pitch early and often to keep White Sox hitters off balance. Throwing the sinker for 73 of his 101 pitches, the most he’s ever thrown in a single start over his career, Manaea generated 40 swings and 10 whiffs (swing and miss) on the pitch. The sinker was also the putaway pitch on eight of his nine strikeouts.

“Being aggressive,” Manaea said when asked what has changed over his past two starts. “Establishing my fastball and throwing it with conviction. I feel like I was getting away from that and worrying about too many different things. I’ve really simplified things and just keep telling myself to be aggressive with all my pitches. Be aggressive with everything and good things will happen.”

Speaking of aggressiveness, that’s what Starling Marte has continuously shown on the bases since joining Oakland via trade on July 28. It paid off on Thursday as he practically generated a two-run third for the A’s on his own.

Reaching first base on a one-out single, Marte swiped second shortly after for what was his Major League-leading 44th stolen base and 22nd since joining Oakland. Following a walk by Matt Olson, Marte’s constant movement off the bag led to White Sox starter Reynaldo López firing off an errant throw to second on a pickoff attempt. Marte scored the go-ahead run on the play while Olson ended up at third. A few pitches later, Matt Chapman’s sacrifice fly scored Olson to increase the A’s lead to two runs.

“He just makes everybody nervous,” Melvin said of Marte. “Whether it’s a ground ball or the potential of a steal. Whether it’s making a pitcher slide step that gets the hitter a better pitch to hit and makes the pitcher’s command a little bit worse. It’s all of the above.

"He’s just not the guy you want on base if you’re the other team, especially in a close game.”

In addition to Manaea finding his groove again, the A’s getting their bullpen back on track is another essential part of the equation in their playoff push. In the past two days, left-hander Andrew Chafin has provided an encouraging development with his handling of the late innings.

After two scoreless innings to finish Wednesday’s victory, Chafin returned and notched his third save of the year with a scoreless ninth. Though he doesn’t bring much experience in the closer role -- Thursday’s save was just the sixth of his career -- Chafin might be the hot hand to ride in such situations. Acquired from the Cubs via trade on July 26, he’s performed well in any role, with 18 scoreless appearances in his 19 games.

“I just try to treat it like every other outing,” Chafin said of closing duties. “Just go out there and get three outs before they score. It doesn’t really matter the inning for me. Just go out there and let it eat.”