De Minaur's semifinal opponent will be second seed Tiafoe, who earned his first trip to the semifinals in Milan by beating the opponent in straight sets 4-2, 4-2, 4-2.
Having been made to work hard for his triumph in Milan, particularly during the second and third sets, the 2018 runner-up was pleased with his powers of recovery.
The Serb will take on Sinner, who went down 4-3 (7-5) 3-4 (3-7) 4-2 4-2 to Ugo Humbert in the final round-robin match.
Australian No. 1 Alex De Minaur secured a spot in the NextGen semifinals on Thursday as he defeated Norway's Casper Ruud with a comfortable 4-1, 4-0, 4-2.
Nadal optimistic on fitness ahead of ATP Finals
I hope he can get it together for 2020 because if I can pick a player that I think that can make it, then it would be Sascha.
"I appreciate this event and I want to continue to be. The more you win, the longer you can stay, so I'm happy to be in the semis".
Matches at the ATP NextGen finals are played on a best of five sets, first to four games in each set basis. I didn't play badly, he just played well in the important moments.
"I think obviously they are doing some great things with trying new rules and innovations here in Milan, and it will be a matter of time to see if any of these stick or we come up with new ideas". You know, that's just me, my opinion. He is the first Italian to make the semi-finals in the tournament's three-year history.
Miomir Kecmanovic eased past Alejandro Davidovich Fokina from Spain 4-1, 4-1, 4-3 (6) in only 58 minutes to set a semi-final clash with Italian wild card entry Jannik Sinner. "It's definitely going to be tough playing against everybody here in the crowd, but I have been playing good".