It was the largest comeback ever in a winner-take-all postseason game, according to STATS, L.L.C. No other club in this sort of ultimate pressure situation had come back from more than four down.
Washington's Bryce Harper tripled during an early burst, then homered the next time up as Gio Gonzalez and the Nationals took an early 6-0 lead.
A night after Jayson Werth capped a 13-pitch at-bat by hitting a winning home run in the ninth inning, the Nationals needed only seven pitches to put the defending World Series champions in early trouble.
Werth led off with a double against Adam Wainwright, Harper tripled, and Ryan Zimmerman homered for a 3-0 lead in the first. Harper and Michael Morse chased Wainwright with home runs in the third.
The 19-year-old Harper became the first teenager to hit a triple in a postseason game - and the second-youngest player to homer.
Down by 6-0, the Cardinals trimmed the deficit with a run in the fourth and two more in the fifth. Still, the two-spot was a letdown given that St. Louis had loaded the bases with none out, with Gonzalez suddenly unable to find the strike zone.
The Cardinals now face the San Francisco Giants on Sunday in Game 1 of the NL Championship Series, in San Francisco.
Werth, who won Game 4 with a ninth-inning shot off reliever Lance Lynn, was greeted with a resounding ovation from the red-clad, towel-waving fans at Nationals Park. He heard the roars again when he lined a double to the left-field corner.
The 19-year-old Harper - 1 for 18 in the series entering the game - followed with a drive to left-center that hit only a foot or two from the top of the wall. Zimmerman then launched a 1-0 pitch to Row 1 in right-center, giving the Nationals a trio of runs before they had made an out.
That was more runs than Washington had scored in the previous two games combined, and it was first time Wainwright had allowed more than one run in any of his 12 career postseason appearances.
Harper left no doubt that he was done with his slump when the all-star put a 3-2 sinker into the right-center stands to lead off the third. Zimmerman then doubled, and Morse came up two batters later and put Wainwright's final pitch of the night into the visitors' bullpen beyond left field.
Wainwright was gone after 21/3 innings, allowing six runs and seven hits. Joe Kelly came on to finish the third and also worked a scoreless fourth. Trevor Rosenthal set the Nationals down in order in the fifth, and Edward Mujica emerged unscathed in the sixth despite allowing two baserunners.
The Cardinals got on the board in the fourth when Carlos Beltran walked and was doubled home by Matt Holliday. The Cards then loaded the bases with none out in the fifth with a double by Descalso, a single by Kozma and a walk to pinch-hitter Shane Robinson.
Gonzalez retired Jon Jay on a soft liner, then bounced a wild pitch in the dirt that allowed Descalso to score. Gonzalez walked Beltran to load the bases again to bring the tying run up to the plate.
Holliday could only manage a tapper in front of the plate, and Kozma was forced at home. Gonzalez issued a bases-loaded walk to Allen Craig - the lefthander's third free pass of the inning - making the score 6-3.