Van Poppel (1-1) toyed with the Phillies, limiting them to three singles and a double over 6 1/3 innings. He walked two and struck out five for his first win since being signed by the Reds in June.
Wily Mo Pena provided all the offense Van Poppel needed, stroking a two-out RBI single in the fourth inning to break a scoreless tie.
Phil Norton worked 1 2/3 scoreless innings before Chris Reitsma retired all three batters he faced and notched his 10th save.
Amaury Telemaco (1-4) pitched effectively for the Phillies, allowing two runs - one earned - and six hits in seven innings with a walk and a season-high eight strikeouts. He also had the Phillies' only extra-base hit, a one-out double in the third.
"I'll bet you one thing - if I pitch like that 10 times with this offense, I would be over .500," Telemaco said. "I trust my guys that they're gonna score some runs. I got faith in each guy."
"He seems to be the guy we don't get any runs for," Phillies manager Larry Bowa said. "When you only get four hits, you're not gonna win too many games. ... They shut us down. Sometimes you got to tip your hat to the pitcher."
The loss dropped the Phillies one-half game behind Florida, which defeated Atlanta in 11 innings, in the National League wild card race.
Telemaco cruised through three innings, striking out five straight and allowing just a single by Van Poppel, before running into trouble in the fourth. With one out, Sean Casey hit a ground-rule double to left and advanced on a groundout by Russell Branyan. Pena followed with a single to left field, plating the game's first run.
The Reds threatened to increase the lead in the fifth, when Ryan Freel reached on a bunt single and took second on third baseman Placido Polanco's throwing error. Juan Castro's groundout moved Freel to third, but Telemaco got D'Angelo Jimenez to fly out.
Cincinnati took advantage of another error by Polanco in the seventh to take a 2-0 lead. Jason LaRue opened the frame with a single and Ray Olmedo followed with a base hit to center. Marlon Byrd fielded the ball and threw to third, but LaRue knocked it out of Polanco's glove as Olmedo advanced to second. Polanco was spiked on the hand on the play when his hand lodged between LaRue's foot and the bag, causing the ball to pop out of his glove. Van Poppel's groundout failed to score the run, but LaRue raced home on Freel's bouncer, beating the throw from shortstop Jimmy Rollins.
Jim Thome drew a leadoff walk in the bottom of the inning and, after Mike Lieberthal struck out, Van Poppel walked Ricky Ledee on four pitches before being lifted. Norton came on and got Rollins to foul out and pinch hitter Jason Michaels to ground out, ending the threat.
After a misadventure in center field on Friday that resulted in an RBI triple for Thome, Freel saved a run in the second inning with an impressive catch. With one on and two outs, Rollins ripped a 1-2 offering into the left-center field gap. Preventing Philadelphia from taking the lead, Freel made a diving backhanded catch to end the inning. Freel later made another diving catch and was joined in the defensive heroics by third baseman Juan Castro who stabbed a potential double down the line and instead, turned a double-play.