"All the pieces we were missing early are finally starting to come together," Bailey said.
Cincinnati slowed Colorado's runaway-freight-train batting order often enough to win two of three games over the weekend.
Bailey (3-2) gave up only Charlie Blackmon's solo homer over 7 1/3 innings and continued to lower his season ERA -- to 4.72 from 6.15 at the end of April. He began his delivery with his hands over his head Sunday, a mechanical sequence he had not used since 2009.
"I needed something to get my timing back," Bailey said. "I had nowhere else to go but better. The next step was to start throwing left-handed."
Bailey eased over a potential fifth-inning speed bump.
The Rockies put runners at second and third with nobody out, thanks to Justin Morneau's leadoff walk and Michael McKenry's ground-rule double that bounded into the left-field seats. Bailey then pulled himself together, striking out DJ LeMahieu and Juan Nicasio and getting Blackmon to pop up and end the inning.
After McKenry's double, Bailey sat down nine in a row before LeMahieu's eighth-inning single.
"To make that kind of adjustment over the course of one start is very impressive," Reds manager Bryan Price said.
Manny Parra finished the eighth inning for Bailey. Chapman, hit in the face by a line drive during Spring Training, entered the game for his season debut. He followed a leadoff walk with strikeouts of Carlos Gonzalez, Nolan Arenado and Morneau to earn the save.
"I was really happy," Chapman said through translator Tomas Veras. "I had a lot of emotion going."
The Reds' batting order, meanwhile, got a bit of assistance from the Rockies.
Billy Hamilton, wielding the pink bat used by many players Sunday as part of the Going to Bat Against Breast Cancer initiative, led off the game by dropping down a well-placed bunt. Morneau's throw was wide of LeMahieu, covering from second base, and Hamilton scurried to third before the ball could be recovered.
Skip Schumaker followed with an RBI groundout to score the game's first run.
Nicasio balked Ramon Santiago from second to third, helping Santiago score on Schumaker's two-out single to center in the third inning. Todd Frazier widened a 2-1 lead to 3-1 with his seventh home run of the season, a solo shot that landed in the grass beyond the fence in center field.
Santiago doubled in the fifth and scored on Brandon Phillips' two-out ground ball with help from Arenado's throwing error.
"We probably could've given up two instead of four," Rockies manager Walt Weiss said.