Nigel Hayes added 11 points and eight rebounds for the Badgers (21-2, 9-1 Big Ten), who are off to their best start since going 21-1 in 2006-7.
Wisconsin overpowered the Wildcats (10-13, 1-9) with its athleticism to open a big early lead.
Dekker started the scoring for Wisconsin with a 3 before later darting through the interior of the 2-3 zone for a dunk. The agile 6-foot-7 forward also converted an alley-oop from Josh Gasser for another slam during a game-opening 21-4 run.
Alex Olah had 15 points for Northwestern. The Big Ten's last-place team extended its losing streak to nine.
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