Edwin Encarnacion drove in two runs for the Reds.
Pedro Feliz hit a two-run homer and Randy Winn added a solo shot, but the Giants didn't score a run after the second inning.
San Francisco had won eight of 10 to get back in the playoff chase after the Giants seemed out of it following their fifth straight loss Aug. 13 at Los Angeles to fall 7 1/2 games behind in the NL West race. The Giants now trail division-leading Los Angeles by 4 1/2 games, and the Giants are five games back in the wild-card chase.
In other games:
* At New York, Shawn Green had an RBI single in his Mets debut and Carlos Delgado homered for the fourth time in four games as New York completed a three-game sweep of the St. Louis Cardinals with a 6-2 victory that extended their winning streak to seven.
Delgado has nine RBI in his last four games, seven against the Cardinals. He had two homers Tuesday night, including a grand slam for the 400th homer of his career.
* At Milwaukee, righthander Tomo Ohka (4-3) drove in a career-high four runs batting lefthanded for the first time in his major league career, and Prince Fielder hit a three-run homer as the Brewers beat Colorado, 12-6, and extended the Rockies' losing streak to six.
* At Pittsburgh, Paul Maholm (6-10) allowed three runs and seven hits in 6 2/3 innings as the Pirates beat Houston, 5-3, sending the Astros to their 10th loss in 13 games. Willy Taveras extended his club-record hitting streak to 27 games with two hits for Houston.
* Braves reliever Danys Baez went on the 15-day disabled list after having an appendectomy.
* The Padres obtained outfielder Russell Branyan from Tampa Bay for minor league pitcher Evan Meek and a player to be named or cash. *