Ruud was supposed to be Sinner's toughest test in Rome since he came back from his three-month doping ban, as the Norwegian came into the match in hot form on clay after winning in Madrid earlier this month.
In his previous matches, Sinner looked to be still finding his feet after his suspension, accepted from the World Anti-Doping Agency for testing positive for traces of clostebol in March last year.
But the 23-year-old demolished sixth seed Ruud in just over an hour with an ominous display of tennis, his domination such that the fans in the packed centre court, seemingly out of sympathy, began to cheer the rare points that Ruud won.
Ruud even got the loudest cheer of the match when he held his serve for the only time, in game three of the second set, holding his arms aloft in ironic celebration.
"I was feeling great on court today. I think we all saw that," said Sinner.
"How I felt today was very, very positive signs for me... I was serving well and also returning well.
"Moving great on the court, so I'm very happy about that."
Tommy Paul, a 7-6, 6-3 winner over Hubert Hurkacz in the day's first match, will have wondered what on earth he can do to stop Sinner if the Italian brings that form to Friday's last-four clash.
Sinner won the first set in just 27 minutes, giving up only seven points as he stalked the court with intent, dressed all in black as he has been all week.
And he then maintained his record of not dropping a set in the tournament to not so much stroll as smash into the last four, and send a message to Carlos Alcaraz.
Spaniard Alcaraz, Sinner's key Grand Slam rival ahead of the French Open, will contest the other semi-final with Lorenzo Musetti on Friday afternoon, with the blockbuster final that tennis fans wanted still on.