HONG KONG, May 21 (Reuters) - Hong Kong shares were set to start weaker on Monday, with the Hang Seng Index pulled down by a 2.2 percent fall in shares of Chinese internet giant Tencent Holdings.

The Hang Seng Index was set to open down 0.26 percent at 18,902. The China Enterprises Index of top mainland listings in Hong Kong was indicated to start up 0.02 percent. (Reporting by Clement Tan; Editing by John Mair)