"Everybody made a concerted effort to try to get the ball from side-to-side and get everybody involved early," said guard Kyle Lowry, who finished with 14 points.
After the ball whipped around the perimeter, DeRozan hit an open 3 from the corner to help build a 77-65 to start the fourth.
But that deficit is nothing for the Bucks, who fell behind by 20 points in each of their first two games on the road, including a rousing 105-98 win earlier in the week in Boston.
True to early season form, Milwaukee stormed back behind frontcourt reserves John Henson (13 points) and Khris Middleton (six), and a 3 from Mayo that tied it at 85 with 5:53 to go.
The buckets stopped falling from there and the defense trailed off.
"Tonight, I thought defensively we weren't tied into together that last four minutes," said Mayo, who said he didn't start after arriving late for practice, and apologized for his tardiness.
Caron Butler, a Wisconsin native playing his first home game since being traded to Milwaukee in the offseason, added 12 points and six rebounds.
The game featured all the typical theatrics of a home opener, with a pulsing bass rattling the arena stands and the Bucks walking individually out of the tunnel with the lights dimmed. The team introduced its new coach and 11 new players.
But all that fun was spoiled by the Raptors.
Nate Wolters, a second-round draft pick, made his first career start at the point because of a hamstring injury to Brandon Knight. He finished with seven points and 10 assists.
NOTES: Besides Knight, the Bucks were also without PG Luke Ridnour (back spasms). ... Raptors G Steve Novak missed his homecoming with a sore back. The seventh-year sharpshooter from the Milwaukee suburb of Brown Deer went to Marquette. He was a freshman on the 2003 team that went to the Final Four.
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