Djokovic pulled out of the tournament after hurting his right knee in an epic last 16 win over Argentine Francisco Cerundolo
Sinner, 22, from South Tyrol, beat Djokovic on his way to the Australian Open title in January and would have become number one anyway if he had made the final.
The Italian star on Tuesday beat Bulgarian world number 10 Grigor Dimitrov to get into the semi-final at Roland Garros against either Spain’s Carlos Alcaraz or Greece’s Stefanos Tsitsipas, respectively world no 3 and 9.
Sinner is the first Italian to get to world number one.
ANSA