Gennett had five hits, including a first-inning home run. Dahl had doubled in two runs in the third and added a sacrifice fly in the fifth. But his liner merely turned into the fifth double play against the Rockies, thanks to Gennett.
"Dahl hit the heck out of that ball and Scooter was right there," Reds interim manager Jim Riggelman said. "It seemed appropriate with the night Scooter had that he would make the play that ended the game. We know we were very fortunate there."
The Rockies maintained a half-game lead on the D-backs, who lost Saturday for the 14th time in the last 16 games. However, the recent struggles of relievers Chris Rusin (0-1) and Bryan Shaw bit Colorado. That is a problem for a club that has struggled offensively and tends to play close games.
The Rockies rallied in the ninth against Jared Hughes when Ryan McMahon, fresh up from Triple-A Albuquerque, led off with an infield single and Chris Iannetta's double to left moved him to third. McMahon didn't have time to score on Iannetta's double, which had an exit velocity of 107.9 mph, according to Statcast™, and caromed to left fielder Scott Schebler.
Did Iannetta hit it too hard?
"I'll never say that," Iannetta said, chuckling.
Charlie Blackmon, whose two hits included an RBI triple, grounded to first baseman Joey Votto, who bounced the throw home but Reds catcher Tony Cruz blocked it and made a barehand pickup before McMahon could score.
Blackmon, whose four hits in the last two games have turned a slump into a hot streak (10-for-33 over the last eight games), lamented that final at-bat.
"That's terrible," Blackmon said. "That's awful. That's the reason we lost the game, in my opinion. No outs, you can tie the game or win the game right there, and you do neither. It's the right situation, the right matchup."
Then came Dahl's 106.5 mph liner.
"They were positioned in the right spot for that particular line drive," Rockies manager Bud Black said. "It was a bullet. We've seen that happen before in the history of the game. And it happened tonight."
Rusin gave up two runs, including a Billy Hamilton game-tying triple, and Shaw allowed two singles, including Jose Peraza's go-ahead single, to turn a one-run lead into a 6-4 deficit in the seventh.
In seven games since returning from a right oblique injury, Rusin has a 10.80 ERA with 12 hits in 8 1/3 innings.
"Coming back from an injury is always tough, especially trying to find a rhythm, trying to find yourself," Iannetta said. "I think he's really, really close. I've been seeing it the last two times out. I think he's right on the cusp of being right there."
Shaw, who has given up six runs and seven hits in his last three outings, was greeted by singles from Peraza and Gennett. Shaw then threw a wild pitch, which scored Peraza.
Nolan Arenado doubled to open the eighth and scored on Gerardo Parra's ground out to cut the defecit to one run, but the Rockies would get no closer.
ANDERSON ADDRESSES CLIP Anderson addressed questions after cameras picked up the lefty intently rubbing his index and middle finger in his glove before throwing a pitch to Gennett.
Anderson was not stopped by the umpires, the Reds didn't complain and the close-up shot cut away from Anderson to Gennett. After the game, Anderson laughed off any notion that he might have been using an illegal substance on the mound.
"It's the same as somebody sitting at their desk, tapping their pen, thinking about something," Anderson said. "I saw the video -- Scooter Gennett hitting. Clearly, the guy's hot right now. He hit a homer first pitch, he hit a couple bullets all over the pitch.
"I'm thinking, 'What pitch do you throw?' The only difference is when you're thinking and you're tapping while you're thinking, people watch it on video and jump to conclusions or whatever. Nobody says anything if you're sitting at your desk, tapping your pen, trying to figure out what you're supposed to write. People do stuff all the time."
HE SAID IT "I did a lot of things right. I tried not to do too much, I felt like I was in control of my emotions, I got a pretty good pitch to hit. I did a lot of things right. He throws sinkers and I know he throws sinkers, but balls usually don't sink that much here and it sank a little more than I thought, and it ended up being a ground ball instead of a line drive up the middle." -- Blackmon, on his ill-fated ninth-inning at-bat
UP NEXT Rockies right-hander German Marquez held the Dodgers to two hits in seven innings while winning his last start on the road, but he has a 10.34 ERA at Coors Field this season. Marquez (3-5, 4.62) hopes to improve that on Sunday afternoon against the Reds and Matt Harvey (1-2, 5.49). In four career starts against the Rockies, Harvey is 2-1 with a 1.88 ERA, but in his only start at Coors Field, he gave up five runs and 11 hits in 5 2/3 innings of a 5-2 loss on May 13, 2016.