The match was the Italians' first of their Asia-Pacific tour.
They continue to Shanghai on Saturday and Melbourne on May 30.
The Serie A giants were missing most of their big-name players and struggled to impose themselves in the opening period.
Santos opened the scoring for the Hong Kong champions midway through the first half and the goalscorer turned provider on the stroke of half-time, setting up Detinho to extend the hosts' lead.
Jonathon Zebina headed home for Juventus two minutes from time.
but South China held out for a famous win.