Ruud made it two wins out of two in the Green Group to seal his semi-final spot with a 6-3 4-6 7-6 (8-6) defeat of Fritz in Turin.
The third seed eliminated Rafael Nadal when he won the first set and in doing so guaranteed that injured 19-year-old Alcaraz will be the youngest player to be at the top of the ATP rankings at the end of a year.
Ruud stormed into a 3-0 lead and did not allow Fritz a way back into the first set, but the American broke for the first time to level the match when his opponent was serving to stay in the second.
The battling Fritz fended off two break points in the fifth game of the deciding set and saved two match points as he fought back from 5-1 down in the tie-break to draw level at 6-6.
Norwegian Ruud was not to be denied, though, becoming the first player to reach the semi-finals when eighth seed Fritz drilled a forehand long at the Palbata Alpitour.
Fritz will do battle with Felix Auger-Aliassime on Thursday (November 17) for a place in the last four.
Stay connected with us on social media platform for instant update click here to join our Twitter, & Facebook
We are now on Telegram. Click here to join our channel (@TechiUpdate) and stay updated with the latest Technology headlines.
For all the latest For Top Stories News Click Here
