
The tall Canadian beat Luxembourg's Muller 6-3, 6-4, 7-6 (7/4) to continue his smooth progress in Melbourne, where he was a semi-finalist last year.
Raonic is largely flying under the radar, and he revealed he was also under the weather during his impressive win, in which his serve went unbroken.
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"Today was just getting through the day," said the third seed, adding he was taking "everything (he) could... Ibuprofen, Paracetamol, something to protect my stomach".
He added: "I just tried to keep it as simple and as much about myself as I could. I tried to really focus on the things that I need to do. Today was sort of the day I tried to play the simplest form of tennis I could really bring."
Raonic, 26, was hampered by injury when he lost to Andy Murray in last year's semis, and he lost again to the Briton in the Wimbledon final.
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His next opponent is the awkward Frenchman Gilles Simon, who took eventual winner Novak Djokovic to five sets in the fourth round last year.
"I've got to serve well and I've got to be aggressive and I've got to take it to him," said Raonic. "The last thing I want to do is get into this sort of game of playing long rallies with him."
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