Oakland, Calif. — Masahiro Tanaka entered Thursday night’s start feeling as good as at any point this season.

Finally pleased with the feel of his signature splitter, Tanaka — likely to be the Yankees’ playoff-opening starter — had ended a poor stretch of four starts with satisfying back-to-back victories.

But things deteriorated quickly for Tanaka on Thursday night at the Coliseum.

Plus, the Yankees’ defense had an off night and their offense was largely missing when it counted in a 5-3 win by the surging Athletics, who swept a three-game series from the AL East leaders.

In the ninth inning, Gleyber Torres belted his second solo homer of the game, adding to his MLB record as the youngest player with the most multi-homer games in one season — now at eight. He has 31 homers for the season.

These teams could meet again in the AL Division Series, with Oakland — winners of 16 of their last 22 games — moving into the AL wild card lead by a half-game.

Having dropped the first three games of this nine-game road trip, the Yankees — who’ve lost four straight overall — now head into a spotlight series at Dodger Stadium.

As the teams with the current best record in each league, the idea of a World Series preview will consume this weekend’s Yankees-Dodgers clash, doubtless played before a Southern California audience dotted by stars and pinstriped fans among the crowd.

But first things first; the Yankees have to — in the words of their manager, speaking on a different subject — tighten some stuff up a bit.

Tanaka’s command issues, and some hard contact by the A’s lineup, led to a three-run first inning.

After a leadoff double by Marcus Semien, Tanaka walked two batters, yielded an RBI groundout and uncorked a wild pitch.

That’s when Mark Canha shot a hard grounder through a drawn-in infield for a two-run single and a 3-0 Oakland lead.

Snaring a comebacker, Tanaka started a double play on Khris Davis to end the inning.

But the Yankees’ defense largely failed Tanaka, when it counted, from that point.

With an ill-advised leap at the wall in the third inning, left fielder Mike Tauchman played Robbie Grossman’s leadoff double into a triple. It was the start of a two-run inning for Oakland.

After Matt Chapman recorded an RBI single with the infield in, Didi Gregorius failed to backhand Olson’s grounder toward the middle, scored as a single.

Tanaka should have been out of the inning two batters later, but second baseman Torres airmailed the relay over first baseman Mike Ford’s head, resulting in an RBI fielder’s choice for Stephen Piscotty and a 5-0 A’s lead.

Despite running a high pitch count early (he threw 109 pitches overall), Tanaka lasted six innings, charged with five runs on eight hits and two walks. He had five strikeouts.

At least, there were improved signs from Aaron Judge and Torres at the plate.

Following an 0-for-5 night on Wednesday, Judge hit the ball hard in his first three at-bats and drew a walk his final time up.

Judge lashed a first-inning double and lined out in the third before singling hard in the sixth.

In fact, Judge and Torres had five of the Yankees’ seven hits off Oakland starter Tanner Roark (8-8), charged with two runs over 6.1 innings. He walked none and struck out seven batters.

Torres singled, doubled and belted a solo homer — his 30th of the season — which ended Roark’s night in the seventh.

After Torres’ fifth-inning double, Tauchman drove him in with a single to left.