Alex Cintron's bases-loaded single finally ended the 3-hour, 51-minute contest. His two-out line shot off a 1-2 pitch from Justin Lehr (0-1) bounced into the visitor's bullpen to send home the scattered few fans still around.
Royce Clayton singled with one out in the 12th, then Tony Clark and Shawn Green walked to load the bases with two outs and bring up Cintron. Brandon Medders (2-0) pitched a scoreless 12th for the victory.
Craig Counsell put Arizona up 1-0 with a leadoff home run in the fifth. The Brewers tied it at 1-1 in the eighth on a balk with a runner at third by Webb, who had blanked Milwaukee on three hits through seven innings.
Webb said that his pitching hand hit his thigh as he went back to throw from the stretch, and he never got rid of the ball.
"It kind of shifted the ball in my hand and I felt like I was going to lose it," Webb said. "I just couldn't make myself throw it in the ground or try to get it to the plate. I just kind of stopped my delivery."
The mistake was especially aggravating because Webb knew his sinkerball was at its best.
"The stuff I had tonight was pretty unbelievable," he said. "I was excited to go out there. I felt like I had some of the best stuff I've had all year."
Damian Miller doubled down the right-field line with one out in the Milwaukee eighth.
Prince Fielder's pinch-hit single put runners at first and third, then Webb stopped halfway through his pitching motion for the balk that brought Miller home.
"I've never seen that before," Miller said.
Third baseman Troy Glaus prevented the go-ahead run with a diving stab of J.J. Hardy's line drive with two outs and a runner at second.
Arizona loaded the bases with two outs in the eighth, but pinch-hitter Chad Tracy grounded into a fielder's choice. Milwaukee loaded them in the ninth with one out, but reliever Tim Worrell got Rickie Weeks to pop out, then struck out Miller looking.
Webb and Milwaukee's Doug Davis each gave up five hits in strong outings.
Webb struck out five and walked one, inducing 16 groundouts in eight innings.
"I thought Webb was tremendous tonight, as good as I've seen him," Milwaukee manager Ned Yost said. "He threw the ball very, very well."
Davis allowed one run, struck out five and walked four in seven innings.
Leading off the fifth, Counsell lined Davis' 2-1 pitch over the right-field fence for his ninth homer of the season, his second in five games. The left-handed batting Counsell's last two homers have come off lefties. Before that, he had only one career home run against a left-hander.
Clayton made a diving stop of Carlos Lee's grounder to shortstop in the fourth, then threw wide to first. Clark had to leave the base to catch the ball, then tried to tag Lee as he ran past. Umpire Laz Diaz called Lee out, despite manager Ned Yost's protest.
Andy Green, who started in left for Arizona, was selected the Diamondbacks' minor league player of the year after batting .343 with 19 home runs for Triple-A Tucson. Green earlier was picked as the Pacific Coast League MVP. ... Davis' 183 strikeouts are the most by a lefty in the NL and third-most in the majors. ... Webb leads the majors with 358 ground balls induced. ... Milwaukee fell a game under .500 at 72-73. ... Weeks was 0-for-5 with three strikeouts.