Cole Irvin’s surprising early-season success is past the novelty stage. Not only has he established himself as a member of the A’s rotation, he’s emerging as the best of the group.

Continuing to flirt with a sub-3.00 ERA, Irvin inched closer to achieving that feat with another strong effort on Saturday. The left-hander twirled 6 2/3 innings of one-run ball against the Twins to lower his ERA to 3.02.

So often this season, such an outing has been finished off with ease by a superb A’s bullpen that entered the day with the best save percentage (12-for-13) in the Majors. But a rare hiccup by left-hander Jake Diekman spoiled Irvin’s shot at a win in a 5-4 series-evening loss at Target Field.

Entering the game in relief of right-hander Yusmeiro Petit with runners at the corners and two outs in an eighth that began with the A’s ahead by three runs, Diekman immediately surrendered a three-run blast to Miguel Sanó on a 1-1 fastball. The homer -- Diekman’s first allowed in 17 1/3 innings this season -- turned a two-run lead into a one-run deficit in a matter of three pitches.

Though Sanó is a right-handed batter, it would be hard to go against manager Bob Melvin’s strategy when bringing in the left-handed Diekman for that spot. Prior to that at-bat, the Twins slugger was 3-for-27 with no extra-base hits against lefties this season.

“They beat our best two guys,” Melvin said. “Every now and then you’re gonna get some balls up.”

The unusual faltering by the bullpen spoiled yet another quality outing by Irvin, who limited Minnesota to just a first-inning run on five hits and two walks with two strikeouts. He’s now allowed two runs or fewer in five of his last six starts, amounting to a 1.89 ERA over that stretch. On Saturday, he displayed the same quality that has allowed him to thrive: pound the zone with strikes.

Entering the middle game of the series with a first-pitch strike percentage of 70.2 -- which ranked fifth-highest among Major League pitchers -- Irvin pumped first-pitch strikes to 22 of the 27 batters he faced and threw a total of 59 strikes on 81 pitches.

That aggressive nature on the mound has drawn the admiration of his fellow teammates, who have watched him grow from an underdog who nobody expected to make the club out of Spring Training into a possible All-Star candidate -- his 3.02 ERA is lowest among Oakland’s starting pitchers.

“It’s been huge to have a guy that goes out there and gives us a quality start every time,” third baseman Matt Chapman said. “He’s going out there and keeping guys off balance. Seems like he hasn’t given up too many base runners, and when he does, he’s able to get ground balls for double plays. It’s good to see him have success.”

Chapman provided Irvin a huge assist in the seventh.

Playing the shift with the bases loaded and left-handed-hitting Luis Arraez at the plate, Chapman quickly shifted to his right on a high pop up hit and raced after a ball in left-field foul ground that appeared ticketed for the stands. Covering 101 feet from his spot near shortstop, per Statcast, Chapman reached over the tarp and secured an incredible catch to end the inning and preserve what was a three-run lead at the time.

Even as a two-time Gold and Platinum Glove Award winner who has made countless highlight-reel plays over the years, Chapman said he surprised himself with his ability to make that difficult play.

“That one looked like it was going further towards the net and I thought I was gonna run into the net,” Chapman said. “I just kept reaching for it and ended up going over the tarp to grab it. I definitely surprised myself a little bit on that catch. I thought I was going to run out of room there and forgot about the tarp.

“Unfortunately, we weren’t able to close out this win. But in that moment, it was definitely exciting.”

Chad Pinder -- who made his first start back from the injured list -- got to second base as the potential tying run with a one-out double in the ninth off Twins closer Hansel Robles. But he was left stranded to end the ballgame.

On a day of strong early offensive production on a pair of homers from Matt Olson and Mark Canha -- marking six homers by five A’s hitters over the past two games -- Saturday’s loss was just Oakland’s third this season when out-homering the opposition, now 20-3 in such scenarios.