Janowicz needed five sets to see off his Austrian foe but came through to set up a quarter-final clash with fellow Pole Lukasz Kubot.
Janowicz who blasted 30 aces past Nicolas Almagro in his previous match did not find things so easy against Melzer. Just 16 aces resulted in five sets before he managed to secure a 3-6 7-6 (7/1) 6-4 4-6 6-4 victory.
The 22-year-old is regarded by most as the biggest danger to Murray in the botttom half of the draw.
He beat the British star en route to the final of last year's Paris Masters, a run which made him one of the players to watch in 2013.
Meanwhile, Kubot banged down 24 aces as, like Janowicz, he made the quarter-finals of a Grand Slam for the first time.
Ranked a lowly 130th in the world, Kubot also had to come from behind to defeat Frenchman Adrian Mannarino 4-6 6-3 3-6 6-3 6-4.