Chapman, who gave up four hits including Matt Dominguez's three-run home run, hadn't blown a save since a June 24 loss against the Twins.
"That shows nobody's perfect. He's been as near prefect as anybody the last few months," said Reds manager Dusty Baker, whose team is trying to lock up the division with 23 games remaining.
The top of the ninth began with all the usual trappings of a normal Chapman appearance. The crowd of 23,785 fans at Great American Ball Park was on its feet when the Cuban lefty crossed the threshold of the bullpen door. First batter Chris Snyder popped out weakly to catcher Dioner Navarro for the first out.
On a 1-1 count, Tyler Greene followed by hitting a 96 mph fastball for a soft single into right field. Pinch-hitter J.D. Martinez fouled off a 1-2 fastball and later a 2-2 fastball. On the next pitch, Martinez blooped a single into no-man's land in shallow center field.
On a 0-1 pitch, a 98-mph fastball, Dominguez launched a drive into the left-field seats.
"I wasn't trying to do too much," Dominguez said. "Everybody knows how good he is. I just happened to put a bat on it and it just happened to go out of the park. Good win for us."
In 63 games, Chapman is now 5-5 with a 1.61 ERA. Between his last blown save and Friday's he had a 0.29 ERA with only one earned run allowed over 30 2/3 innings.
"It's tough to take but Chappy's human and he's been great for us," Baker said.
Had the Reds won the game, the shortcomings of the earlier innings would have been overlooked. After the loss, the missed chances became more glaring.
In the bottom of the first inning vs. Astros starter Lucas Harrell, the Reds had the bases loaded with no outs. Ryan Ludwick rolled a two-run single through the left side that made it a 2-2 game, but the Reds could not add on in the inning. Jay Bruce flied out on a drive to center field and the inning ended when Todd Frazier grounded to shortstop for a double play.
"They're playing us tough," said center fielder Chris Heisey, who extended his hitting streak to seven games in a 3-for-5 night. "They have a young group of guys. Maybe they're not motivated because they're not going to make the playoffs. But they're motivated because they're trying to get jobs for next year. So they're busting their butts and playing hard. We just have to take advantage of opportunities when we get them."
In the fifth, Heisey drove a one-out triple to the wall in left-center field against Harrell. Later, with the bases loaded on back-to-back walks, Bruce broke his bat while popping up to shallow left field. Shortstop Greene extended his glove but just missed the catch as the ball dropped for a single that scored Heisey. The blooper extended Bruce's hitting streak to seven games.
Frazier followed by grounding to the pitcher. Harrell threw home to Snyder, who fired to third base as Ludwick slipped and fell trying to get back for double play.
"It was a situation where we could have blown the game open early too," Baker said. "That's something we've been haunted with most of the year. We have to score those runs when we need them and don't even get in that situation, hopefully."
Reds starter Homer Bailey gave up two runs and five hits over his 6 2/3 innings with one walk and nine strikeouts. Bailey's lone blemish came with two outs in the Houston first inning when Justin Maxwell crushed a 3-2 pitch an estimated 433 feet to center field for a two-run homer.
Bailey sidestepped trouble the rest of the way. Despite his own errant pickoff throw in the third, he left runners on first and second in the third and runners on the corners in the fourth. Bailey needed only six pitches to cruise through a perfect fifth inning.
Baker lifted Bailey after Dominguez's two-out single on pitch No. 117.
Sean Marshall and Jonathan Broxton each pitched two-thirds of an inning to set it up for Chapman. This time it wasn't an easily slammed door as Bailey was burned by a blown save for a staff-high sixth time this season.
"Our bullpen has been so solid all year," said Bailey, who remained 4-0 with a 1.46 ERA in eight career starts vs. Houston. "Chapman is one of the best in the game right now. Sometimes, that's just the way it goes. There's nothing you can do about it."