Volquez (17-6) gave up a leadoff home run to Kazuo Matsui in the first inning, then checked the Astros on four hits over the 6 1/3 frames for his first win since Aug. 29.
The loss dropped the Astros 4 1/2 games behind the Mets in the National League Wild Card standings with six games to play. The Astros' elimination number is two, meaning any combination of New York wins and Houston losses totaling that number will eliminate them from the playoff picture.
"We had to win, so it's definitely a tough one coming down the stretch," said pitcher Chris Sampson. "We couldn't get it done tonight, and it's very disappointing."
While they still are mathematically alive, the numbers are daunting. Not only would the Astros essentially have to win out, they also would need the two teams ahead of them, the Mets and Brewers, to collapse.
"Definitely a disappointing loss," Houston manager Cecil Cooper said. "We let our golden opportunity slip by tonight."
As good as Volquez was, the Astros made it easier on the gifted right-hander by squandering a scoring opportunity in the sixth.
With one out and Michael Bourn on second base, Houston's Miguel Tejada singled to left field. Bourn stopped at third, and Tejada was caught in a rundown. Bourn stayed there as Volquez, after walking Lance Berkman intentionally, got Geoff Blum to line out to first to end the threat.
"In that scenario, you don't use a coach," Cooper said. "When you're a baserunner with that kind of speed, with a broken-bat single, you should score standing up. Michael made a mistake, and he said he thought he was being stopped and he did not stop."
Bourn shouldered the blame for the baserunning gaffe.
"We could still be playing if I hadn't stopped," Bourn said. "I saw two hands go up when I was coming around third, and it confused me a little bit. I didn't know what to do, but I take the blame for it. I know I'm supposed to be scoring in that situation."
Third-base coach Ed Romero had been waving Bourn around, and Tejada was urging him from the other side of the diamond.
"When I hit the ball, as soon as I hit the ball, I knew there was going to be a close play at home," Tejada said. "When you face a guy like Volquez -- he's one of the best pitchers in the game -- you know it's going to be hard to score runs. You can't go hit by hit by hit. That's why I went to second. I tried to make something happen.
"He throws 98 mph with a nasty changeup and a great slider. I mean that guy pitches like Cy Young. You face a guy like Volquez, you've got to make the extra [things], because hit by hit you're not going to beat him. He's so tough he can strike out the next three guys."
The game was delayed seven minutes in the top of the seventh inning when the umpires went to review video of a ball hit by Cincinnati's Joey Votto. With the Reds leading, 2-1, Votto hit a drive to right field that appeared to touch the top of the wall before clanking off a first-row seat.
Votto thought he had a home run, but the play was ruled a single by first-base umpire Bill Miller. Votto, Reds first-base coach Billy Hatcher and manager Dusty Baker argued the call.
After all four umpires deliberated for a several moments, crew chief Gary Darling went to the third-base dugout to review the play. After a seven-minute delay, Darling confirmed Miller's call, Votto had to settle for a single and play resumed.
The Astros had a runner in scoring position in the ninth, but Ty Wigginton lined into a game-ending double play. Wigginton's liner was speared by shortstop Paul Janish, who fired to second baseman Jeff Keppinger to double off Reggie Abercrombie. Second-base umpire Jerry Meals initially called Abercrombie safe, but after conferring with the rest of the crew, Abercrombie was called out.
"He was out," Cooper said. "The umpire said he couldn't tell. He had to duck out of the way a little on the play. He couldn't tell, so he asked for help. Nothing we can do about that one."
The Reds scored a pair of unearned runs off Wandy Rodriguez (8-7) in the first inning.
"I thought Wandy threw the ball really well, considering he'd been out for a while. I was really concerned about his command coming in," Cooper said.
In a quiet Astros clubhouse after the game, Tejada and his teammates looked toward Wednesday.
"[The loss] is pretty tough," Tejada said. "I want to win. We're in a situation right now where we need to win. Tomorrow's another game. We'll see what happens."
Jim Molony is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.