The No.1 seed won just two points across the first three games but took a first set that featured four breaks and lasted more than an hour.
Duckworth broke in the first game of the second but eventually ceded that advantage and the frustrated Australian could not prevent Sinner from breaking again to claim a 7-6 (7-4) 6-4 win to set up a last-four clash with Filip Krajinovic, who prevailed in three sets against Kamil Majchrzak.
Gael Monfils will feature in the other semi-final after brushing aside Gianluca Mager 6-2 6-2, booking a meeting with American Giron.
It is a second semi-final in as many weeks for Monfils, who also reached the last four in Metz.
Giron will contest his first Tour-level semi-final after a 6-4 6-2 defeat of another Australian in John Millman.
"Nothing is easy," Monfils said. "We have been putting in work for nearly four days here. Working on my serve and my forehand and it worked perfectly today. I will be ready for Marcos. He has been playing very well, so I think it will be a tough semi-final."