HOUSTON -- The Red Sox entered Monday with the Majors’ highest bullpen ERA in August, but one silver lining was star closer Kenley Jansen. In 5 2/3 perfect innings, the 36-year-old hadn’t allowed a baserunner, and he hadn’t surrendered a hit since July 27.

That silver lining -- and Jansen’s streak -- is no more. After Yordan Alvarez struck out to start the ninth on three elevated cutters, Astros catcher Yainer Diaz yanked Jansen’s next high cutter for a towering Statcast-projected 401-foot home run. That tiebreaking shot rallied Houston to a 5-4 victory in the opener of a three-game set at Minute Maid Park.

“I made three really good pitches on Alvarez, and unfortunately that one [to Diaz] wasn't well executed,” said Jansen. “Give credit to the hitter. He ambushed me, and it won that game. It's tough. Tip your cap. But we've got to move on and fight tomorrow. Let that be motivation.”

“That's baseball,” said fellow reliever Lucas Sims, who took a blown save when Mauricio Dubón roped a two-out, game-tying RBI single off the righty in the eighth. “It's a beautiful game, but it's tough sometimes.”

With Monday’s loss and a win by Kansas City, which defeated the Angels, the Sox (65-59) fell 4 1/2 games behind the Royals (70-55) for the third and final AL Wild Card spot.

“I look at the standings … and the scoreboard all the time,” said manager Alex Cora. “I know Kansas City won. I watch. I'm a baseball fan, and I know where we're at. But I'm confident in where we are, pitching-wise. If we continue to do that, we'll be OK. Our starters are throwing extremely well.”

Cora found confidence in All-Star starter Tanner Houck, who kept his season ERA at 3.01 with a third straight quality start. On Monday, he scattered eight hits and three runs (two earned) over six innings while striking out eight. With the potential tying run on with two outs in the sixth, Houck ended his night by inducing a Dubón flyout on the third of three consecutive 97 mph fastballs -- all among his hardest-thrown pitches this season.

After being blasted by Texas (five innings, six runs) in his first August start, Houck has responded in a big way. That has included two starts vs. Houston in which he’s given up only three earned runs in 12 innings.

“I’m just throwing a lot more strikes, and living in the zone more frequently,” Houck said. “Putting myself in pitcher-advantaged counts, the numbers don't lie. It puts you at such an advantage.”

Cora also found confidence at the plate, where the Sox took advantage of four uncharacteristic Astros errors in the first six innings to score four runs while grabbing two separate two-run leads. Lefties Jarren Duran and Masataka Yoshida each delivered opposite-field homers to Houston’s Crawford Boxes, and Yoshida’s two-run shot came with two outs in the sixth to give Boston a 4-2 lead.

It was the first pinch-hit blast of Yoshida’s Major League career, and it came after the Astros pulled lefty starter Yusei Kikuchi (5 2/3 innings, six hits, three runs, one earned) for righty reliever Tayler Scott. That prompted Cora to insert Yoshida for Connor Wong, and the Japanese star took advantage.

“He's locked in,” Cora said. “We had talked a little bit. He was sitting right next to me during the game. I said, ‘You feel good, right?’ He said, ‘I feel great.’ This is the guy we saw when we signed him. The discipline has been unreal, and he's driving the ball all over the place.”

But despite those early Houston miscues and Boston’s hitting heroics, the bullpen again faltered, and it came after seemingly being in control. After having two outs and none on with a one-run lead in the eighth, Sims gave up an opposite-field bloop single to Chas McCormick, who promptly stole second base. From there, Dubón tied it.

In the ninth, Jansen started off by dominating Alvarez -- who had reached base in his first four plate appearances -- with a three-pitch strikeout. But one pitch later, Houston turned the tables, and that exhilaration immediately turned to misery.

In some ways, it’s a script that is becoming too familiar for the Sox, who are now 0-4 versus the Astros. But it’s one they’re poised to try and fight through.

“You can't focus on the past,” Jansen concluded. “The past will make you miserable, and the future will give you anxiety. You've got to stay in the moment. Even that homer, that's in the past.

“If we want to be in the playoffs and try to win a championship, we can't feel sorry for ourselves. We've got to fight. We've got to have urgency.”