Sinner struggled with illness in his previous match but looked sharp in a 6-3 6-2 6-1 victory over eighth seed De Minaur.
Two days after being sick before playing Denmark’s Holger Rune and needing to see a doctor, Sinner said he felt “ready” when he woke up on Wednesday.
Sinner denied speculation he had pneumonia, saying blood tests after his fourth-round match were “all good”.
“I feel like the illness has gone away now. I was feeling much, much better this morning,” the world number one said.
“When you are young you recover very fast – so it’s a bit different.”
The 23-year-old Italian will face American Ben Shelton in the last four on Friday.
Shelton, 22, reached the Melbourne semi-finals for the first time with a battling 6-4 7-5 4-6 7-6 (7-4) against Italy’s Lorenzo Sonego.
