WASHINGTON — Doesn’t matter how bad things look for the St. Louis Cardinals. Trailing by a bunch, down to their last strike, they stay calm and do what it takes to win.

Erasing a six-run hole in Game 5 slowly but surely, the defending World Series champion Cardinals got a tying two-out, two-run single from Daniel Descalso and a go-ahead two-run single from Pete Kozma in the ninth inning and came back to beat the Washington Nationals 9-7 Friday night and win their NL division series.

It was the largest comeback in a winner-take-all postseason game, according to STATS LLC. No other club in this sort of situation had come back from more than four down.

First-year manager Mike Matheny and the wild-card Cardinals, the last team to clinch a playoff spot this year, will open the NL championship series at San Francisco on Sunday.

Down 7-5 with two outs in the ninth against Nationals closer Drew Storen, the Cardinals twice were a strike away from losing. But Storen walked both batters, Yadier Molina and David Freese, setting the stage for the unheralded Descalso and Kozma — Nationals manager Davey Johnson called the rookie “Cosmos” before Game 4 — to come through.

When Cardinals closer Jason Motte got Ryan Zimmerman to pop out to second base a half-hour past midnight, the Cardinals streamed from the visiting dugout.

Over the past two years, St. Louis is 6-0 when facing elimination, including victories in Games 6 and 7 of the 2011 World Series against Texas.

Washington, which won the NL East and led the majors with 98 wins, got off to a fantastic start.

Seven pitches, three runs. Just like that, Jayson Werth’s double, Bryce Harper’s triple and Zimmerman’s homer jump-started the hosts.

A big third inning highlighted by the 19-year-old Harper’s homer, made it 6-0.