Michael Lorenzen (1-0) pitched one inning in relief of Tyler Mahle to earn the win, and Jared Hughes survived the ninth for his third save.
Gennett tied a career high for hits, set last year against St. Louis, and used some solid defense to help the Reds secure the win.
The Rockies loaded the bases with no outs in the ninth off Hughes, but after a force out at home, Gennett snared David Dahl's liner near second base and then doubled up Carlos Gonzalez to end the game.
Dahl doubled and had three RBIs, and Charlie Blackmon and starting pitcher Tyler Anderson scored twice each for the Rockies.
Chris Rusin (0-1) faced only two batters but allowed two runs on two hits to take the loss.
Gennett hit the first pitch he saw from Anderson into the Cincinnati bullpen to give the Reds a 1-0 lead in the first. Colorado rallied to take a 2-1 lead in the third on Dahl's double off the wall in center.
Cruz gave Cincinnati the lead right back in the fourth with his first homer of the season, a two-run shot to left that went an estimated 449 feet.
The Rockies took a 4-3 lead in the fifth when Charlie Blackmon had an RBI triple and scored on Dahl's sacrifice fly.
Cincinnati got to Colorado's usually reliable bullpen in the seventh. Scott Schebler led off with a single and Billy Hamilton tripled to center off Rusin.
Bryan Shaw relieved Rusin and allowed an RBI single to Jose Peraza to put the Reds ahead 5-4. Gennett's single put runners on the corners, and Peraza scored on a wild pitch to make it a two-run game.
Nolan Arenado led off the eighth with a double off David Hernandez, moved to third on a wild pitch and scored on Gerardo Parra's groundout.
--Field Level Media
--Field Level Media
Cincinnati | Colorado | |
Tyler Mahle | Player | Tyler Anderson |
No Decision | W/L | No Decision |
5.0 | IP | 6.0 |
4 | Strikeouts | 2 |
6 | Hits | 6 |
7.20 | ERA | 4.50 |
Cincinnati | Colorado | |
Scooter Gennett | Player | Chris Iannetta |
5 | Hits | 2 |
1 | RBI | 0 |
1 | HR | 0 |
8 | TB | 3 |
1.000 | Avg | .667 |
Team | Hits | HR | TB | Avg | LOB | K | RBI | BB | SB | Errors |
Cincinnati | 11 | 2 | 19 | .306 | 14 | 4 | 5 | 3 | 0 | 0 |
Colorado | 11 | 0 | 16 | .344 | 18 | 6 | 5 | 5 | 0 | 0 |