Jake Westbrook has won 82 games in the major leagues on the strength of one of the best sinkers in baseball.
Ryan Doumit drove a curveball into the right-field stands for a three-run home run in the bottom of the fourth inning, breaking a 1-1 tie and sending Westbrook (9-7) to his first loss in four starts. Doumit went 4 for 4, matching a career high with his sixth four-hit game.
St. Louis, which had won three of its last four, fell six games behind Milwaukee in the NL Central. The Brewers blanked the Los Angeles Dodgers 3-0 on Monday night.
Westbrook (9-7) gave up five runs, four earned, and nine hits in 5 2-3 innings while walking one and striking out three.
"I wasn't really controlling the counts and on top of that they were fouling a lot of pitches off and running up my pitch count," Westbrook said. "The last couple of times I've been that way I've been able to settle in and have a couple of quick innings but it just kind of caught up with me. That was a big mistake to Doumit, though, and it really cost us."
St. Louis went 2 for 13 with runners in scoring position and left eight runners on base.
"The name of the game is execution," Cardinals manager Tony La Russa said. "We had a double twice and didn't score a run. A lot of that is their pitching but some of it was we could have done a better job."
Xavier Paul, who homered, and Neil Walker each had two hits for the Pirates, and James McDonald (8-6) pitched 5 2-3 uneven innings to end his three-start winless streak. McDonald allowed two runs and six hits while walking two and striking out four.
Jose Veras got what was likely the biggest out of the game for Pittsburgh when he relieved Daniel McCutchen in the seventh. With two outs and runners on first and second with a 5-2 lead, Veras got Albert Pujols to look at a called third strike on a curveball on the inside corner.
"It mano-a-mano right there," said Doumit, the Pirates' catcher. "You know Albert is coming up ready to do some damage but Jose has great stuff with his fastball and curveball. We decided to go with the curve to keep him off balance and get him out, and it worked."
Pujols went 1 for 4 with a single one night after taking over the NL home run lead with 29. He is looking to hit 30 homers for the 11th consecutive season.
Paul opened the scoring when he led off the bottom of the first inning with his second home run of the season and third of his career.
The Cardinals tied it in the third as Rafael Furcal's two-out double drove in Theriot, who led off with a single and advanced to second on Westbrook's sacrifice bunt.
Doumit homered in the fourth to put the Pirates ahead. Walker and Garrett Jones led off the inning with singles and Doumit followed with a drive to right field.
David Freese's RBI single chased McDonald in the top of the sixth but Pittsburgh got the run back in the bottom of the inning on a throwing error by shortstop Furcal.
Walker doubled home a run in the seventh to push the Pirates' lead to 6-2.
At six games back, St. Louis is now facing its largest deficit of the season as it tries to catch Milwaukee, which is 11-2 in August.
"We've just got to keep going," Theriot said. "We're getting some pitching right now and we just have to score more runs than the other team."
Notes: Pirates LF Jose Tabata, on the disabled list since June 27 with a strained left quadriceps, returned to Pittsburgh to be examined by team doctors after hitting .333 (11 for 33) with six doubles and two RBIs in nine games with Triple-A Indianapolis on a rehabilitation assignment. If cleared by doctors, Tabata could be activated Tuesday. ... St. Louis RHP Edwin Jackson is expected to make his next scheduled Saturday against the Cubs at Chicago. A cramp in his right hamstring forced Jackson out in the sixth inning of Sunday night's victory over Colorado. ... St. Louis signed OF Charlie Tilson, their second-round draft pick from New Trier (Ill.) High School, beating the midnight deadline for agreeing to terms with drafted players. ... Chris Carpenter (8-8) starts for St. Louis against Jeff Karstens (9-6) on Tuesday night in the middle game of the three-game series. Carpenter is 12-2 with a 2.24 ERA in 17 career starts against Pittsburgh and has also gone 7-1 with a 2.60 ERA in 10 starts since June 20. Karstens has won six of his last eight decisions.