The SAR drew first blood in the Guangdong-Hong Kong Cup football competition.
Goals from Chan Siu-kwan and Poon Pui-hin in each half gave Hong Kong a 2-0 win in the first leg at the Mong Kok Stadium in front of more than 5,000 fans.
The return leg is in Guangzhou next Wednesday.
This is the 42nd edition of the Guangdong-Hong Kong Cup, which resumed for the first time since 2019.