Karstens threw a five-hitter and Andrew McCutchen tripled and drove in two runs as the surging Pirates opened the second half of the season with a 4-0 victory over the Astros on Friday night.
With the win, the Pirates moved into a tie for first place with the St. Louis Cardinals, marking the latest in a season the team has been in first place since 1997.
Karstens (8-4) needed just 83 pitches to become the first Pirate to win five straight decisions in a season since Matt Capps in 2006. He has an ERA of 2.34 this year but has been particularly dominant in his three starts against Houston, going 3-0 with an 0.41 ERA.
"I had an idea it was kind of low, but I didn't know how low it was," Karstens said of his pitch count. "It's just one of those things where I'm just trying to go out there and be efficient and see where it takes me at the end of the night."
It was the fewest pitches thrown in a complete game this season and just the sixth time since 2002 that a pitcher has thrown a complete game with 83 pitches or less, according to statistics provided by the Pirates from the Elias Sports Bureau.
It was the second career complete game and shutout for Karstens, who last did it in 2008, at Arizona.
"It's one of the best pitched games I've seen," Pittsburgh manager Clint Hurdle said. "You take into account the pitches thrown, 21 out of 30 first-pitch strikes, and then you get the leadoff hitter six out of nine innings ... defense helped him out a couple times, but that was a masterful outing. You talk about throwing strikes, pounding down in the zone, changing speeds, he had it all working tonight."
McCutchen raved about Karstens.
"He was real efficient and got an easy win, pretty much," he said. "He made it look easy, anyway."
Houston starter Brett Myers (3-10) allowed five hits and three runs while striking out a season-high 11 in six innings in his fourth consecutive loss.
McCutchen's two-run triple to center field came in the third inning and scored Presley and Chase D'Arnaud to make it 3-0. McCutchen picked up where he left off before the All-Star break and has 10 RBIs in his last five games.
Houston manager Brad Mills liked the way Myers got back on track after his rough start.
"After the third inning, I thought he threw the ball extremely well, a lot like he threw last year," Mills said. "To see the six strikeouts out of the 11 hitters he faced after the triple in the third inning, that was pretty good."
Karstens sailed through the first five innings, facing the minimum thanks to a double play in the second inning after Hunter Pence led off with a single.
"My whole goal when I'm on the mound is to get my team off as quick as possible, get them back in to hit," Karstens said. "A lot of times when you do that, you look up and it's the sixth inning."
Pinch-hitter Chris Johnson singled with two outs in the sixth inning on a grounder to right. Michael Bourn followed with a single on an infield dribbler that Karstens couldn't corral, but Matt Downs grounded into a forceout to allow Karstens to escape the jam.
Jeff Keppinger singled in the seventh inning but was out at second when Pence grounded into a forceout. Pence stole second base and Carlos Lee singled. Karstens was able to get out of the inning when Brett Wallace grounded into a double play.
The struggling Astros tried to spark their offense with a lineup shuffle on Friday. They moved Lee out of the cleanup spot for just the second time this season and batted Pence fourth with Lee hitting behind him. It didn't help as the Astros, who have the worst record in the majors, dropped their fifth straight and were shut out for the fifth time this season.
"We are trying to figure him out a little bit better," Mills said of Karstens. "He's been throwing the ball extremely well, probably gaining confidence as he goes along."
Garrett Jones walked in the eighth inning and was replaced by pinch-runner Xavier Paul, who advanced to second on a wild pitch before scoring when Lyle Overbay reached on an error by Keppinger to make it 4-0.
The Pirates recalled left-hander Joe Beimel (left elbow inflammation) from the 15-day DL on Friday. Pittsburgh optioned left-hander Daniel Moskos to Triple-A Indianapolis on Thursday to clear a spot on the roster. ... Friday was the 15th time in his career that Myers finished with double-digit strikeouts. ... Houston outfielder Jason Bourgeois (right quadriceps strain) went 0-for-3 on Friday in his rehabilitation assignment with the Gulf Coast League Astros. He is scheduled to join Double-A Corpus Christi on Saturday.