ST. LOUIS -- Rookie Tyler Mahle maneuvered his way through five scoreless innings Wednesday night to pick up his first big league win as the Cincinnati Reds damaged the St. Louis Cardinals' playoff hopes with a 6-0 decision at Busch Stadium.
Eugenio Suarez's fifth-inning grand slam was the game's biggest hit.
Mahle (1-2) scattered four hits, walked three and fanned four in his fourth start since being called up last month from Triple-A Louisville. Three relievers completed the shutout as Cincinnati improved to 63-83.
It was just the third loss in 12 September games for St. Louis, but it was an ill-timed setback. The Cardinals (76-69) fell 2 1/2 games back of the first-place Chicago Cubs in the National League Central and into a second-place tie with the Milwaukee Brewers. St. Louis also fell four games behind Colorado for the NL's second wild-card spot pending the Rockies' late result.
Jack Flaherty (0-1) absorbed the loss in his third big league outing. Flaherty lasted 4 1/3 innings, giving up four hits and three runs with two walks and three strikeouts.
For the second straight game, Cincinnati started the scoring with a first-inning homer. This time, it was rookie Jesse Winker on the second pitch with a towering blast down the right field line, his fifth home run of the year.
The Reds expanded their lead with five runs in the fifth, thanks to a replay reversal that turned an apparent inning-ending forceout by Zack Cozart into an infield single when it was ruled that Mahle beat the flip to second by diving shortstop Paul DeJong.
Joey Votto poked a first-pitch single to left to make it 2-0, and Suarez blasted a grand slam to left, his 25th homer of the year, for a 6-0 advantage.
Mahle worked around traffic through most of the first five innings but didn't allow a run. St. Louis stranded eight runners overall, including seven in the first three innings.
NOTES: St. Louis activated 3B Jedd Gyorko (right hamstring) from the 10-day disabled list. Gyorko will be available for the time being as a pinch hitter. He was injured Aug. 26 while rounding third base against Tampa Bay. ... Cincinnati 2B Scooter Gennett (left hand inflammation) did not play. Gennett left the Tuesday night game before the bottom of the fifth inning after injuring himself while hitting a foul ball in the fourth. ... Cardinals CF Dexter Fowler (left knee) isn't sure when he'll be able to return from an injury sustained in a collision with the wall in the seventh inning Saturday night. He missed his third consecutive start.