"I knew I hit it good enough," Navarro said. "He came out of nowhere and made a spectacular play, unfortunately for us."
The Reds were stymied by Milwaukee starter Yovani Gallardo (10-8), and trailed 5-1 when beleaguered reliever Francisco Rodriguez entered the game to open the eighth. Rodriguez again struggled as the Reds loaded the bases with one out, prompting the Brewers to bring in John Axford.
Pinch-hitter Xavier Paul greeted Axford with a single to make it 5-2, and Navarro stepped to the plate. With the speedy Paul at first as the tying run, the Reds catcher worked a 3-2 count before lofting the ball toward the gap.
Aoki sprinted in front of the warning track and made a sliding catch for the second out, with Scott Rolen tagging and scoring on the sacrifice fly to pull the Reds within 5-3.
"Those plays, they turn games around," Milwaukee manager Ron Roenicke said. "By shutting an offense down when you make a play like that is big."
Cincinnati starter Bronson Arroyo (7-7) matched Gallardo for the first four innings, but the Brewers scored a run in the fifth to snap their scoreless streak at 21 innings.
"The game was going along fine," Arroyo said.
In the sixth, the home run came back to haunt the right-hander. Aramis Ramirez, Corey Hart and Martin Maldonado each homered, allowing Milwaukee to open a 5-0 lead.
"It happened quick," Cincinnati manager Dusty Baker said. "He was sailing along pretty good."
Arroyo said he simply threw too many pitches up in the strike zone.
"Keeping the ball down was going to be at a premium today," Arroyo said. "You could just tell, the ball, especially going to right-center, was just flying out. Some balls that even we hit looked like they were going to be lazy fly balls ended up being to the wall."
The two-run home run by Maldonado, who also had a pair of doubles on a three-RBI night, appeared to be a long fly ball, but it kept carrying and just cleared the fence near the left-field foul pole for a 5-0 lead.
"I wanted to keep it there at three [runs]," Arroyo said. "It would have given us a chance to win the ballgame, which it would have. But that's the way it goes."
Both Arroyo and Baker said they were surprised the ball cleared the fence.
"I didn't think that was going out," Baker said. "We all thought that was a popup. Next thing I knew it was over the fence. It was his night."
Baker said he hoped to keep the score close because he knew Gallardo's pitch count was high and the Milwaukee starter likely would be taken out of the game before too long.
"He's probably coming out the next inning, and they've been having a little trouble in the bullpen," Baker said. "It's a big inning for them. The next inning we were threatening, big time."
The loss, combined with Pittsburgh's 4-0 win over Arizona, dropped the Reds' lead to 3 1/2 games over the Pirates in the National League Central.
After the Reds fell behind, 5-0, Zack Cozart started the attempted comeback with a solo home run in the seventh, his 12th this year.
Arroyo allowed a Major League-leading 46 homers last season but had given up only 17 in 21 starts this year. The last team to hit more than two in a game off the right-hander was Milwaukee, which hit four in a 6-3 victory over the Reds in Cincinnati on Sept. 16, 2011.
Arroyo's personal three-game winning streak was snapped. The right-hander pitched 5 1/3 innings, giving up five runs on 10 hits while striking out five.
"They put some good swings on some balls, and they played a great game," Arroyo said of the Brewers.
The Reds couldn't manage clutch hits when facing Gallardo, going 0-for-6 with runners in scoring position against Milwaukee's starter. Axford recorded the final five outs for his 18th save.
Brandon Phillips returned after missing five games with a strained left calf and went 0-for-5.