The 35-year-old Nishikori, who reached a career-high four in the world a decade ago and was a US Open finalist in 2014, was 4-3 up when Shang called it quits in their semifinal.
The 19-year-old has been suffering with a fever.
Nishikori, who was the first Grand Slam men's singles finalist from Asia, faces France's Alexandre Muller for the title on Sunday.
It is the Japanese player's 27th tour-level final but first since winning in Brisbane in 2019.
Nishikori has emerged from years of injuries, including major hip surgery, to roll back the years in Hong Kong.
A wildcard at the tournament, he beat third-seeded Russian Karen Khachanov, Britain's Cam Norrie and Denis Shapovalov on the way to the last four.
The Japanese player will return to the top 100 for the first time since June 2022 on Monday.
Nishikori is a 12-time ATP winner and a 2016 Rio Olympic bronze medallist.
