San Diego Padres' Yonder Alonso, left, is tagged out by St. Louis Cardinals catcher Yadier Molina, right, during the fourth inning of their baseball game, Tuesday, Sept. 11, 2012, in San Diego. (AP Photo/Gregory Bull)
SAN DIEGO (AP) — The San Diego Padres are turning out to be the last team a playoff contender wants to play.
Logan Forsythe and Alexi Amarista had key hits in a five-run fourth inning, and the Padres beat the St. Louis Cardinals 6-4 Tuesday night, sending the slumping World Series champions to their 10th loss in 14 games.
St. Louis maintained a one-game lead over the Los Angeles Dodgers for the NL's second wild-card berth but dropped 10½ games behind Cincinnati in the NL Central.
The Padres, who have won 16 of 21 overall, have captured six of their last seven series with their two wins over the Cardinals. San Diego is 10-4 against teams with winning records since Aug. 17.
"We're playing good ball right now," Forsythe said. "Big hits in big situations. We're having fun."
Forsythe, a homer shy of the cycle in Monday night's 11-3 win over St. Louis, doubled in two runs and Amarista added a two-run triple as San Diego erased a 2-0 deficit in the fourth.
"If we play a good ballgame against teams that are fighting for a (playoff) spot, it's a great learning experience and let's them know how they measure up," Padres manager Bud Black said.
Edinson Volquez (10-10) allowed three runs — two earned — and four hits in 5 1-3 innings, striking out six and walking six. He has a career-high 100 walks, the most in the majors this season.
Luke Gregerson pitched 1 2-3 innings for his sixth save in 10 chances, finishing San Diego's 16th win in 21 games.
Adam Wainwright (13-13), hurt by rookie shortstop Pete Kozma's error in the fourth, gave up five runs — two earned — five hits and four walks in six innings. He had been 3-1 with a 1.07 ERA in seven career appearances against San Diego.
St. Louis was just 1 for 14 with runners in scoring position and stranded 13 runners. The Cardinals have lost seven of their last eight road games.
"The bottom line is we have to win, regardless of if the other teams in contention are losing or not," Cardinals third baseman David Freese said.
"This is a good opportunity to lengthen our lead in the standings and we are not doing it. It's putting more pressure on this team and allowing other teams to stick around," he said.
With St. Louis ahead 2-0 in the fourth, San Diego loaded the bases with one out when Kozma threw wildly on Will Venable's grounder, and Chase Headley and Yasmani Grandal reached on an infield single. Yonder Alonso walked on a full-count pitch, forcing in a run, and Forsythe hit a two-run double to right-center for a 3-2 lead.
After Alonso got thrown out at home trying to score on Cameron Maybin's grounder to second baseman Skip Schumaker, Amarista hit a two-run triple into the right-field corner.
Jon Jay's RBI single in the sixth off Joe Thatcher and Freese's seventh-inning homer against Brad Brach pulled the Cardinals to 5-4. Cameron Maybin had an RBI single off Edward Mujica in the eighth.
St. Louis loaded the bases with no outs in the second but scored only one run, on Schumaker's groundout. Volquez got out the jam by getting Kozma to ground into a double play. Shortstop Everth Cabrera's error on Yadier Molina's grounder allowed Jay to score in the third.
Alonso, who walked three times, hit his 34th double of the season in the eighth, breaking the San Diego rookie record set by Benito Santiago in 1987.
NOTES: Wainwright's only other loss to the Padres came at San Diego on May 25, 2010, when he tied a career high with 12 strikeouts over seven innings in a 1-0 loss. ... Wainwright had walked just 43 batters in his previous 28 starts over 173 2-3 innings. ... Padres LF Carlos Quentin, who was removed from Monday night's game, was out of the starting lineup Tuesday with a sore right knee. ... Cardinals RHP Kyle Lohse (14-2, 2.81 ERA) opposes Padres LHP Clayton Richard (12-12, 3.78) in the series finale Wednesday.