Sinner, back in action this week for the first time since January's Australian Open win following a three-month doping ban, saw off Ruud 6-0 6-1 on Thursday.
The 23-year-old Italian produced a remarkable display against the world No.7 and Madrid Open champion, who had been on a nine-match win streak, to blast to the win in just 63 minutes.
Asked how close his victory was to perfection, Sinner said: "It is tough to say, but I was feeling great on court and we all saw that.
"My goal this week is to understand where my level is, and it has raised day by day.
"Everything can change in one day, and one performance doesn't tell you everything about my shape at the moment but I am very happy. I moved great on the court. Now let's see what is coming in the semi-finals."
Sinner will now face Tommy Paul after the American 11th seed made it to the last four in Rome for the second year in a row, beating Poland's Hubert Hurkacz 7-6 (7-4) 6-3.
But Paul's mind has not entirely been on tennis, with the 27-year-old telling reporters about his distress at having his precious truck repossessed.
"On court everything has been going pretty well. Off court, a little bit of a stressful week," he explained.
"I got my truck repossessed. I missed a couple of payments and they took my truck from my house this week. I've been grinding to get my truck back. If you know me, that's my baby.
"I'm excited to get it back. I had to win a couple of matches so I could pay that off."
Paul, who lives in Florida, revealed he had paid 1000 US dollars to secure the vehicle's return, sending his trainer to pick it up while he battled on the European clay.
But Sinner will be the overwhelming favourite to emerge victorious from the semi-final clash with the man who knocked out Australian Alex de Minaur in the last-16.
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