Phillips and Ryan Ludwick had three hits apiece and Jay Bruce added a two-run homer as Cincinnati regained a seven-game lead over second-place St. Louis, which rallied to win the series opener 8-5 on Friday night.
Leake (6-8) allowed at least one hit in every inning except one, but kept the Cardinals mostly at bay. Leake, who also had two hits, lasted 6 2-3 innings, allowing 10 hits and two runs.
Cincinnati grabbed control with three runs in the sixth inning. Phillips led off with a double down the right-field line and advanced on Ludwick's single to right. Phillips scored when Todd Frazier beat out a potential double-play grounder, and Bruce followed with a 432-foot drive to right for his team-best 27th homer.
The Reds added four more in the seventh after Brandon Dickson retired the first two batters of the inning.
Phillips drove a full-count pitch just over the wall in right-center for his 14th homer and first since July 31 against San Diego. Scott Rolen's two-run single made it 7-2, and Dioner Navarro chipped in with an RBI single that dropped into left when Matt Holliday lost the ball in the sun.
The Reds grabbed the lead in the third against Jaime Garcia, making his second start since June 5 after missing 64 games with a left shoulder strain. Leake hit a leadoff double and scored on Phillips' two-out single into right field.
The Cardinals got one hit in each of the first four innings against Leake before finally pushing across a run in the fifth. Tony Cruz, Rafael Furcal and Jon Jay singled to load the bases with one out, setting up Matt Carpenter's tying sacrifice fly.
St. Louis added another run in the seventh when Cruz scampered home on Jay's double-play ball.
Garcia (3-5) allowed four runs and seven hits in six innings. He entered with a 7-1 record and a 3.12 ERA in nine career starts and two relief appearances against the Reds.
NOTES: Cardinals manager Mike Matheny said RHP Joe Kelly will start on Wednesday in Pittsburgh in place of RHP Lance Lynn, who hasn't lasted more than five innings in any of his last four starts. Lynn pitched two-plus innings in St. Louis' win on Friday. ... Cardinals C Yadier Molina and 3B David Freese were out of the lineup due to injuries. Molina rolled an ankle a few days ago, Matheny said. Freese injured his left wrist fielding in a play at third base on Friday. ... The Reds retired Barry Larkin's No. 11 during a pregame ceremony. Larkin was inducted into baseball's Hall of Fame on July 22. ... The Great American Ball Park flags in left-center field were lowered to half-staff to honor the memory of astronaut Neil Armstrong, the first man to walk on the moon. Armstrong passed away earlier Saturday.
Link to boxscore: Cincinnati 8, St. Louis 2