Sinner barely gave Ruud time to breathe as he rattled off the first three games and took the first set in half an hour, before sweeping to a comprehensive victory.
Fifth-ranked Fritz triumphed 6-3, 3-6, 7-6 (7/3) over world number two Zverev in a thrilling match at the season-ending event in Turin.
Carlos Alcaraz said he was hoping to make Rafael Nadal's tennis farewell at the Davis Cup Finals a victorious one after he was dumped out of the ATP Finals at the group stage with a straight-sets defeat to Alexander Zverev on Friday.
Alexander Zverev secured a spot in the last four of the ATP Finals on Friday with 7-6 (7/5), 6-4 victory over Carlos Alcaraz which leaves the Spaniard's destiny out of his hands.
Sinner was in no mood to disappoint his passionate home supporters and put in the latest in an increasingly long line of impressive performances to head into the semis with a perfect three wins from three.
Former Uruguay striker Diego Forlan swung and missed on court with Argentina's Federico Cori as his tennis doubles debut ended in defeat, losing 6-1 6-2 to Bolivia's Boris Arias and Federico Zeballos at the Uruguay Open on Wednesday.
World number four Daniil Medvedev said the ATP Tour calendar needs to be refocused around the Grand Slams and Masters 1000 events with the lower-tier ATP 250 tournaments moved to an optional slot later in the year.
The small furry objects that are the focus of every tennis player's energy are spinning the stars at the ATP finals into a fury and leaving them deflated.
Czech Barbora Krejcikova on Sunday called for respect and professionalism in sports media as she hit out at "unprofessional" remarks made about her appearance during a broadcast on the Tennis Channel.
Poland's Iga Swiatek will not successfully defend her WTA Finals title after being knocked out of the season-ending tournament in the group stage on Thursday.
Former Manchester United and Atletico Madrid striker Diego Forlan said Tuesday that he felt "like a little boy" after swapping his football boots for a tennis racket at the age of 45.
Novak Djokovic said on Tuesday he will not defend his ATP Finals title due to an "ongoing injury" which has ended the 24-time Grand Slam champion's season.
Former world number one Maria Sharapova and the US doubles team of Bob and Mike Bryan were announced on Thursday as 2025 inductees to the International Tennis Hall of Fame.
The 26-year-old jumped above Poland's French Open champion Iga Swiatek thanks to year-end adjustments in the standings.
Novak Djokovic implored long-time adversary Rafael Nadal to put off his planned retirement after defeating the Spaniard Saturday in what was likely to be the final chapter of their "amazing rivalry".
The 38-year-old Nadal announced last week he would retire from tennis after the Davis Cup Finals in Malaga next month.
Nadal, the winner of 22 Grand Slam titles, announced last week that he will retire from tennis after the Davis Cup Finals in Malaga next month.
Novak Djokovic insisted Sunday that he plans "to compete and play next season" despite suffering another loss at the hands of world number one Jannik Sinner.