Bruce hit a three-run home run to cap a five-run fourth, leading the Reds to a 6-1 victory over the Brewers on Monday night.
Estrada (0-2) used 43 pitches to set the Reds down in order each of the first three innings but gave up five runs in just 11 pitches to open the fourth.
"It's like a feeding frenzy," Cincinnati manager Dusty Baker said of his team's quick outburst.
Bronson Arroyo (2-1) pitched solidly into the seventh inning, giving up only a homer to Ryan Braun in the first -- the NL MVP's ninth. The right-hander was tough the rest of the way, giving up six hits and a walk with nine strikeouts in 6 2/3 innings.
Bruce's ninth home run extended his hitting streak to 10 games and gave the Reds a comfortable lead against Estrada.
"The first time around, he obviously cruised," said Bruce, who is batting .405 (15-for-37) with six home runs and 12 RBIs during the streak. "We got him in the zone a little better the next time around and put some good swings on some balls. We can hit as a team. We're a good hitting team."
Cincinnati has won six of eight and is 3-1 in a stretch in which it will play 13 of 16 on the road.
Milwaukee has lost five of six, scoring only 12 runs in that span. Aoki went 0-for-4, and the Brewers' leadoff hitters are batting .162 (18-for-111) this year.
The Brewers played hours after learning that shortstop Alex Gonzalez tore his anterior cruciate ligament in his right knee Saturday and will miss the rest of the season. Milwaukee first baseman Mat Gamel (torn ACL in right knee) and fifth starter Chris Narveson (torn left rotator cuff) are also out with season-ending injuries.
"It was a different lineup than what you expect against these guys, with the amount of guys they had hurt," Arroyo said. "I didn't have a lot of reps against four or five of those guys, so I was feeling my way through the game."
Estrada, making his third start in place of Narveson, pitched seven innings, giving up five runs on eight hits with five strikeouts. He retired the first nine batters he faced before falling apart in the fourth.
"He threw a great game except for that inning," Milwaukee manager Ron Roenicke said. "I don't know why we're having those tough innings, but he threw the ball really well the rest of the game."
Joey Votto went into an 0-2 hole but hit the sixth pitch from Estrada down the right-field line for a double, scoring Stubbs and giving the Reds a 2-1 lead.
Brandon Phillips followed with a first-pitch single to right, moving Votto to third.
Bruce then hit a 1-0 pitch deep onto the right-center field concourse, giving the Reds a 5-1 lead.
"It's like boom, boom, boom, boom," Baker said. "It's the game of baseball. You go to the bathroom and come back, there could be four or five runs on the board. That's how quickly it happens."
Milwaukee put runners on second and third with no outs in the fourth, and loaded the bases with two outs in the inning. Cesar Izturis, starting in place of Gonzalez, popped out to end the inning.
"Getting out of that fourth inning was huge," Arroyo said. "It changed the whole momentum of the game."
Pinch hitter Miguel Cairo's ground-rule double scored Chris Heisey from second to make it 6-1 in the ninth.
NOTES: The victory was Baker's 1,499th. He is 1,499-1,380, 20th all-time for wins by a manager. ... Before the game, the Reds activated INF Cairo from the 15-day DL and outrighted Willie Harris to Triple-A Louisville. The transaction removes Harris from the 40-man roster. He can refuse the assignment and become a free agent, or he has 72 hours to report to Louisville. ... Cozart has a five-game hitting streak.