Djokovic is trying to become the second man after Roy Emerson to win five Australian Open titles. He's also aiming for his eighth major title overall.
The top-seeded Serb hasn't lost a set yet in the tournament. He was rarely in trouble against Muller, saving all four break points he faced and hitting 47 winners to just 16 unforced errors.
He next faces eighth-seeded Canadian Milos Raonic, against whom he has a 4-0 head-to-head record.
The last time Djokovic failed to reach the quarterfinals of a Grand Slam was at the 2009 French Open.
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