The 44-year-old Clemens will have two more scheduled starts this season -- and possibly forever -- at Philadelphia on Monday and at Atlanta on Sept. 30.
Clemens won't say what he'll do beyond that.
"It's definitely a challenge physically and mentally every year," he said. "If I commit to come back and play, I want to be able to perform.
"I don't know what tomorrow brings -- but if this is it, I've really enjoyed it."
The seven-time Cy Young winner is sure about one thing -- if he ever starts to feel like he can no longer dominate, that's when he'll retire for good.
"I can't handle being average," said Clemens, who lowered his 2006 ERA to 2.37. "I guess there's a time when you know and I thought that was three years ago. But I had to come back and really challenge myself to get it done."
Luke Scott had a three-run triple and a solo homer and Craig Biggio hit a three-run homer as the Astros won for just the third time in 10 games, a slide that has left them with fading hopes in the wild-card race. The Astros are 5½ games behind the Los Angeles Dodgers and Philadelphia, the NL wild-card leaders, with 11 games left and also trail San Francisco and Florida.
Clemens (7-5) struck out six and allowed three hits with three walks. He did not yield an earned run for the fourth time this season and allowed fewer than three for the 15th time in 17 starts.
Before he threw his first pitch, catcher Brad Ausmus met him at the mound and thanked him for a memorable 2½ seasons with the Astros.
"I personally hope that it isn't it," Ausmus said, "but catching Roger has been a highlight of my career. He's the ultimately prepared professional and certainly a first-ballot Hall of Famer."
Clemens walked out for the seventh, but Astros manager Phil Garner came out before he threw a pitch and replaced him with Russ Springer.
The fans stood for a roaring ovation and Clemens tipped his cap as he walked off the field.
"He wasn't going to pitch anymore," Garner said, "but the way things played out, everything came together for that moment. The fans' reaction was unbelievable and Roger's reaction to that was just as special."
The cheering continued and Clemens came out for a curtain call, shirt untucked, pumping his fist in the air before disappearing into the dugout.
"That's what it's been about since I've been home," he said. "Obviously, winning and making it enjoyable for our fans at home."
Reds starter Bronson Arroyo (14-10) lost for first time in five starts, giving up seven hits and striking out four in 5 1/3 innings.
The Reds have also slipped from view in the wild-card race, entering the day five behind the leader.
Scott, who had a double, triple and home run in Tuesday's 5-4 loss to the Reds, hammered Arroyo's first pitch of the second inning into the upper deck in right field. Scott's eighth homer of the season was his fourth since Sept. 9.
The Reds have also slipped from view in the wild-card race, remaining five behind Los Angeles.
Scott, a 28-year-old rookie, then lined a 3-2 pitch into the right-field corner, clearing the bases with his sixth triple.
Clemens struck out two Reds each in the second, third and fourth and allowed two harmless singles through the first five innings.
"You look out there and you say, 'How's this guy still putting up zeros?"' said Arroyo. "You take some of his velocity away and normally a guy starts to slip. But he's out there throwing cutters and split-fingers. It's amazing what he can still do."
Ray Olmedo hit a one-out single in the sixth and Clemens walked Adam Dunn, reaching 100 pitches for the ninth time this season. But Scott Hatteberg and David Ross flew out to end the inning and the crowd, sensing Clemens was done, gave him a standing ovation.
Adam Everett walked and Ausmus beat out an infield single leading off the Astros' seventh. Clemens came out to bat and the fans stood and cheered again before Clemens laid down a sacrifice bunt.
Biggio followed with a homer to left, his 20th of the season, and Reds manager Jerry Narron replaced Arroyo with left-hander Bill Bray.
- Biggio hit his 280th career homer.
- CF Ryan Freel and RF Chris Denorfia left the game after three innings after Freel sprained his left wrist and Denorfia cut his left ankle. They were replaced by Norris Hopper and Todd Hollandsworth.
- Clemens led off the Astros' third with a double down the left-field line, his second hit of the season.