Jon Lester matched a season high with nine strikeouts on Tuesday. (AP)

For eight innings, it looked like precisely the kind of victory upon which the Red Sox could build as they concluded the first half of the season. Instead, they suffered a sudden and startling 3-2 defeat to the A’s when closer Alfredo Aceves proved unable to secure the 2-1 lead with which he’d been entrusted, allowing a pair of ninth-inning runs as the Sox dropped a second straight to the lowly A’s.

The ninth-inning culprits were twofold. First, there was Aceves, who allowed three hits in two-thirds of an inning of work while blowing a save for the fourth time in 23 opportunities this year.

Then, there was Ryan Kalish, who was thrown out trying to steal third for the third out of the ninth inning and then committed a fielding error in the bottom of the ninth while trying to pick up the game-tying single, a misplay that placed the winning run on third base (instead of second), thus permitting former Sox outfielder Coco Crisp to drive in the go-ahead run with a sacrifice fly.

The Red Sox are now 2-4 on their West Coast swing.

WHAT WENT WRONG FOR THE RED SOX

• Aceves, who had converted each of his last eight save attempts and 17-of-18 since April 23, was not hit hard, but three singles, Kalish’s error and a sac fly were enough to cost the Sox the game.

• Cody Ross endured an 0-for-4 night in which he struck out three times.

• Nick Punto went 0-for-4, a hitless night that included a failed bunt attempt that resulted in a double play when he popped up in an attempt to deliver a sacrifice bunt with runners on first and second and no outs in the ninth, resulting in not only his own out but also a double play when Mike Aviles was thrown out at first by first baseman Chris Carter.

• Ryan Kalish was thrown out trying to steal third in the top of the ninth for the third out of the inning, in a situation where a single likely would have scored him from second. He then made it worse with an error in center, continuing his shaky defensive play in his return to the majors this season.

WHAT WENT RIGHT FOR THE RED SOX

• Jon Lester delivered a dominant start for the Sox. Though he gave up a homer to Coco Crisp leading off the bottom of the first, Lester was overpowering for the rest of the contest, putting his team in position to win until Aceves’ blown save.

In 6 2/3 innings, Lester gave up just the lone run while scattering four hits, walking one and striking out nine, matching his season high. At one point, he retired 12 straight batters and 17-of-18. Though his fastball velocity was mostly 89-92 mph on the night, he featured an impressive off-speed arsenal, foremost a swing-and-miss changeup. He continued a run in which he has been relentless in attacking the strike zone, striking out 47 and walking just six over his last 47 innings spanning seven starts.

The outing nudged Lester’s ERA down to 4.33, still far beyond his typical standards. Even so, for a Red Sox team that is desperate for signs of promise that the left-hander remains a top-of-the-rotation talent, Tuesday’s start fit the bill in a way that few other contests this year have done so.

• Vicente Padilla continued his outstanding work as a setup man. He inherited a runner from Lester with two outs in the seventh and, naturally, stranded him, as he’s now left 18-of-19 inherited runners on base this year. He then returned for the eighth, when he loaded the bases with one out but rebounded to strike out Yoenis Cespedes and Jonny Gomes with the bases loaded to keep the lead intact. The 1 2/3 shutout innings extended Padilla’s scoreless streak to 12 innings spanning 13 games.

• Jarrod Saltalamacchia continued to be a lineup force. He unloaded on an 84 mph changeup, blasting it into the right-field bleachers for his 16th homer of the year. At the halfway point of the season, Saltalamacchia is on pace for 32 longballs this year, a number that would shatter the longstanding Red Sox record for homers by a catcher in a season. Carlton Fisk is the record-holder with a pair of 26-homer campaigns, one in 1973 and another in 1977. Saltalamacchia leads all major league catchers in homers as well.

• Adrian Gonzalez went 2-for-4 to extend his hitting streak to 14 games. He has just three extra-base hits (two doubles and a homer) during the run (both of his hits on Tuesday were singles), but for a hitter who has been struggling for much of the year, a 14-game line of .356/.387/.441/.828 represents progress.

• David Ortiz went 1-for-3 with a single and a walk, the first time in six games on the road trip that he reached base twice in the same game.