BOSTON — No one has been able to drive the 2021 Boston Red Sox into extinction, yet. The Yankees tried and failed. Tampa Bay had its chances, but could not do it, either. It almost happened much earlier, during the final weekend of the regular season in Washington, D.C., but Boston survived.

Now it is the Houston Astros’ turn. With one more win they can end Boston’s bumpy, unexpected ride across their seven-month season, as the Red Sox sit, once again, on the precipice of doom.

On Wednesday, Houston pounded the suddenly slumping and sloppy Sox, 9-1, in Game 5 of the American League Championship Series in front of a somber crowd at Fenway Park, grabbing a three-to-two advantage.

Boston’s offense, which had blasted three grand slams in a span of 11 innings over two games and piled up 25 runs in the first three games of the series, turned frigid beginning on Tuesday. In Game 4, the Red Sox scored only twice. In Game 5, Framber Valdez, Houston’s starting pitcher, set down the first 12 Boston batters without allowing a ball out of the infield and did not allow a run through the first six innings.

Valdez turned in the longest pitching performance of the entire postseason, going eight innings and allowing only one run, three hits and a walk while striking out three.

Before Game 4, Astros pitchers had been serving as Boston’s punching bag. But in the last two games, Houston’s starters and relievers held the Red Sox scoreless over 14 consecutive innings — from the second inning of Game 4 until a home run by third baseman Rafael Devers in the seventh inning of Game 5 on Wednesday — seizing momentum in the series.

It is almost as if the teams changed places sometime overnight Monday. Houston has assumed the mantle of the team of destiny. It has the more potent offense and the impenetrable pitching staff, outscoring Boston by 18-3 in a 24-hour Boston nightmare.

The Red Sox went into the game looking for Chris Sale to match Valdez and provide a strong outing to tip the balance back in their favor. But the 2021 Chris Sale has not been the same dominating pitcher he once was, struggling to regain his form since his return this year from Tommy John surgery in March 2020. He pitched fairly well in the regular season after his return in August, but in the postseason he has been an utter disappointment.

For a while on Wednesday it looked as if he had rediscovered his old stuff. After he struck out Kyle Tucker for the final out of the fourth inning to strand Houston runners at first and third, Sale yelled emphatically into his glove as he danced off the mound, celebrating the high, 98.5-mile-per-hour fastball that eluded Tucker’s bat.

Houston had grabbed an early lead on a bases-empty home run by Yordan Alvarez in the second, and then lashed out five more runs in the sixth while Fenway fell silent. Alvarez also had a two-run double in the sixth that ensured Sale’s exit. The results of that inning will likely fuel questions about whether Red Sox Manager Alex Cora left Sale in too long against an awakening and thunderous offense that had produced the most runs of any team in 2021.

The Astros are still perhaps the most hated team in baseball, a group reviled across the continent for a cheating scandal that has dogged them since it was uncovered in 2019. But they are also still very good, and with one more win they will go back to the World Series for the third time in the last five years, whether America is ready for that, or not.

The next two contests are in Houston, and history favors the Astros. Of the previous six times the A.L.C.S. was tied 2-2, five of the teams that won Game 5 went on to win the series. The only team that lost Game 5 and still made it to the World Series was the 2017 Astros, who lost Game 5 to the Yankees but won the next two.

That was the year the Astros used illicit methods to steal opposing teams’ signs. They won the World Series by beating the Los Angeles Dodgers, but were later punished for their misdeeds. Jeff Luhnow, the team’s general manager at the time, and A.J. Hinch, its manager, were fired and replaced by James Click and Dusty Baker.

The core of that team is still the same, with Jose Altuve, Carlos Correa and Alex Bregman forming a dynamic group that helped the Astros win 95 games and the A.L. West. Most of their marquee pitchers from the past, including Justin Verlander, Gerrit Cole and Charlie Morton, are gone, but the Astros have found a way to advance, beating the Chicago White Sox in their division series.

After Boston grabbed a 2-1 lead in the A.L.C.S., optimism flourished in Boston that this scrappy team of overachievers would somehow get back to the World Series, even after trading away Mookie Betts, their best player, in 2020. Houston’s pitching was in tatters, Red Sox fans thought. Boston’s offense had unleashed a record-setting onslaught and could not be suppressed. The World Series awaited.

But all that has dramatically changed — at least for now. The Red Sox have demonstrated an uncanny ability to avoid mortality in 2021. Houston has two chances to do what no other team has been able to do yet: Put the 2021 Red Sox away for good.