Sinner dropped serve in the middle of the second set, but soon re-established his advantage to wrap up victory over Shelton, 23, inside 70 minutes.
“At the moment I’m not thinking about the ranking,” added the Australian Open and Wimbledon champion. “It’s a consequence of how I’m playing.
“Making the semis here in Paris means a lot. It has been a very long season with some great, great results and I never take these results for granted.”
Sinner will face Zverev in the last four after the German fought back to beat Medvedev 2-6 6-3 7-6 (7-5).
Alexander Bublik will appear in a Masters semi-final for the first time after winning 6-7 (5-7) 6-4 7-5 against Australian sixth seed Alex de Minaur, sparking emotional scenes for Kazakhstan’s number one.
The 13th seed will next face Canada’s Felix Auger-Aliassime, with the ninth seed having ended the run of Shanghai Masters champion Valentin Vacherot with a 6-2 6-2 success.