TOKYO, Jan 6 (Reuters) - ** Japan"s rubber futures slid on Thursday, reversing the previous day"s gain, as an anticipation of faster policy tightening by the U.S. Federal Reserve dented risk appetite, while worries over the surge in COVID-19 cases weighed on sentiment. ** The Osaka Exchange rubber contract for June delivery , was down 3.9 yen, or 1.6%, at 239.9 yen ($2.1) per kg as of 0318 GMT. ** The rubber contract on the Shanghai futures exchange for May delivery was down 180 yuan, or 1.2%, at 14,660 yuan ($2,301) per tonne on Thursday. ** A "very tight" job market and unabated inflation might require the Federal Reserve to raise interest rates sooner than expected and begin reducing its overall asset holdings as a second brake on the economy, U.S. central bank policymakers said in their meeting last month. read more ** Growing concerns over the rapid spread of the Omicron coronavirus variant also hurt market sentiment. read more ** The front-month rubber contract on Singapore"s SICOM exchange for February delivery last traded at 175.2 U.S. cents per kg, down 0.3%. ($1 = 115.9500 yen) ($1 = 6.3714 Chinese yuan renminbi) Reporting by Yuka Obayashi; Editing by Shounak Dasgupta