The Cardinals fell to 81-76 on the season.
Manager Tony La Russa, when asked if he had anything to say to the Reds, said "Congratulations."
Since a victory against the Cubs on Aug. 13 that put them one game ahead of the Reds, the Cardinals have won 16 games and lost 27.
The Pirates (56-101), who improved to 16-60 on the road this season, scored three times in the first off Cardinals starter Jeff Suppan (2-8).
Andrew McCutchen led off with a walk and scored on Neil Walker's grounder to first base. Walker reached on an error by Suppan and then first baseman Garrett Jones connected on his 21st home run, a two-run shot to center field, to put Pittsburgh ahead, 3-0.
"It was up," Suppan said. "He's a type of guy that can get his hands extended. I was trying to have some depth on the changeup and it just stayed up."
Cardinals rookie catcher Matt Pagnozzi blasted a solo home run off the facade of Big Mac Land in left field in the second inning. It was the first home run of Pagnozzi's Major League career and came in his 28th career at-bat.
"It feels great, of course," Pagnozzi said. "Unfortunately, it came in a game where we didn't come out on top, so I'm kind of bummed about that."
St. Louis added a run in the third, when rookie third baseman Daniel Descalso scored on an error by Pirates shortstop Ronny Cedeno.
That was the last run the Cardinals would score on the night.
St. Louis hammered Burres in his last start, on Sept. 23 in Pittsburgh, when it tagged him for four earned runs on six hits in 2 1/3 innings. But on Tuesday, Burres (4-4) shut down the Cardinals and lowered his ERA from 5.29 to 5.01 with the victory.
"Last time, I got into a little bit of trouble and couldn't get out of it," he said. "This time, I made some better pitches and the defense made some good plays behind me."
Four Pirates relievers -- right-handers Chris Resop, Sean Gallagher, Evan Meek and Chris Leroux -- combined to limit the Cardinals to one hit over the final 3 1/3 innings, while striking out five and walking none.
The Pirates chased Suppan from the game with one out in the sixth. He allowed four runs (three earned) on five hits in 5 1/3 innings. He struck out three and walked one.
"Looking back, I think I had some good movement," he said. "I think my location struggled a little bit tonight. I think in some situations, I was trying to be too fine. Obviously, you want to stay out of the big inning and tonight that happened with the home run. Just trying to make pitches after that to get outs, basically."
La Russa said: "He's a craftsman. He did a good job. He did a good job. He kept us in the game."
Pittsburgh led 4-2 in the sixth after an RBI single by Jones off reliever Trevor Miller, and then pushed that advantage to 7-2 on John Bowker's pinch-hit, three-run double with two outs in the eighth inning off Mike MacDougal.
Miles, the Cardinals utilityman, tossed a scoreless ninth inning in relief. It was his fifth career pitching appearance, his second of the season, and the fourth scoreless appearance of his career.