Pujols' solo shot was his fifth homer in three days -- and second in as many at-bats. He pulled a flat 76 mph curveball from Pedro Astacio over the left-field fence on a full count.
"I'm seeing the ball good and putting on my best swing, and that's how it is," Pujols said. "When they come, they come in bunches. When they don't come, they take a while, so I just try to put on my best swing and I'm just glad they are going out of the park."
Suppan (11-7) allowed five hits, struck out five and walked two over 7 2/3 innings, his second scoreless outing in three starts. He blanked the Chicago Cubs for 7 2/3 innings on Aug. 25.
"These guys are pretty tough hitters and they were patient," Suppan said. "But when they were patient I was able to locate pitches, and when they were aggressive I was able to throw chase pitches. For me, it's pitch by pitch, so that's how I try to keep my focus. I think location was obviously a big key."
Suppan left after Ryan Zimmerman walked and Nick Johnson singled with two outs in the eighth. Adam Wainwright entered and hit Austin Kearns to load the bases before striking out Brian Schneider, ending another fruitless day for the Nationals when batting with runners in scoring position. Washington was 0-for-7 with men on second and third, dropping the team's season average to .245.
Astacio (3-5) took the loss despite pitching well after three poor outings. He allowed two runs and six hits over 6 1/3 innings and didn't walk a batter, his best performance since throwing a shutout against Atlanta on Aug. 15.
"He did all he could do," Washington manager Frank Robinson said. "We just didn't score any runs for him."
The Cardinals got their second run in the seventh when Nationals second baseman Bernie Castro dropped a throw from shortstop Lopez on a potential double-play ball with the bases loaded. Scott Rolen scored, but the rally stopped when Suppan, the next hitter, lined into a double play while attempting a squeeze bunt.
The Nationals' five-game winning streak came to an end, but their incredible streak of comeback victories had the Cardinals on edge late in the game. Washington won the first four in its spurt after trailing by two runs in the seventh inning or later in each game. The Nationals had the bases loaded in the eighth and two men on in the ninth on Tuesday.
"You look at the scoreboard and you see it's the eighth inning," Schneider said. "It's funny how things have been working out like that for us, but not tonight."
- The Nationals played their first night game since Saturday. They played a day-night doubleheader Saturday and had matinees Sunday and Labor Day. They will also play a getaway afternoon game Wednesday.
- Zimmerman's hitting streak ended at 10 games.
- St. Louis CF Juan Encarnacion sat out for the second straight game with a sore hip.