Mitchell Boggs (0-1), the stand-in closer for St. Louis, yielded six runs while getting only one out and the Cardinals needed four more pitchers to get out of the inning. Only a few thousand fans remained of the largest regular-season crowd of 47,375 for the bottom of the ninth in 8-year-old Busch Stadium history.
The defending NL Central champions Cardinals held a tribute for Stan Musial before the game.
Phillips added his 150th career homer for the Reds, who have won six of seven since losing in extra innings to the Angels on opening day and totaled 11 or more hits the last four games. Jay Bruce had four hits and Chris Heisey doubled twice with an RBI.
Sam LeCure (1-0) pitched an inning for the win.
Cardinals lefty Jamie Garcia matched his career best with 10 strikeouts in 6 2-3 innings and left with a 4-3 lead. Yadier Molina hit both of the fly balls dropped by Choo in the first and sixth, and added an RBI single.
Pinch-hitter Xavier Paul tied it in the eighth with an RBI single off Trevor Rosenthal before the Reds took off in the ninth with six hits, five walks and an error. The Cardinals last surrendered nine runs in the ninth in a 12-9 loss at Colorado July 6, 2010.
Prior to the game, Musial's four children unveiled an oversized red No. 6, the Hall of Famer's retired jersey number, on the wall in left-center. The decal matches memorial patches worn on the team's uniform sleeves.
Reds players stood on the top dugout steps as the Cardinals paraded around the warning track in flat-bed trucks, and manager Dusty Baker gave a hug to Hall of Famer Ozzie Smith.
Choo entered the season with 652 games of outfield experience but only 10 in center, and all 155 games with the Indians last year as the right fielder. Two runs scored in the first when Choo twice juggled Molina's fly ball after backtracking, and Matt Holliday scored from first to put the Cardinals up 4-2 in the sixth when the ball popped out on Choo, perhaps fighting the sun.
Reds starter Mat Latos allowed one earned run in six innings, shaving his ERA to 10.23 in five career starts at Busch to 10.23.
Garcia struck out four of the first six batters and retired eight in a row before issuing a four-pitch, two-out walk to Latos in the third. He entered with a 2.48 career ERA at Busch, the stadium's best, and his other 10-strikeout game was at home Aug. 19 against the Pirates.
NOTES: The Reds totaled two errors their first six games while their opponents had 12. ... Jason Isringhausen and Jim Edmonds, teammates on the Cardinals' 2006 championship team, threw first pitches to injured closer Jason Motte and Adam Wainwright.
Link to box score: Cincinnati 13 at St. Louis 4