The BSE benchmark Sensex rallied by almost 197 points in opening trade on Friday, extending gains into the fourth straight session on sustained buying by funds, driven by firming trend overseas. The 30-share index of the Bombay Stock Exchange rose by 196 points, or 1.07 per cent, to 18,547 in opening trade. The Sensex has gained nearly 512 points in the previous three sessions. Stocks in the IT, realty banking, auto and metal sectors, led the rally. Similarly, the broad-based National Stock Exchange Nifty index also gained 66.30 points, or 1.20 per cent, to 5,588. Japan's Nikkei Index was up by 1.38 per cent and Hong Kong's Hang Seng Index was up by 0.81 per cent in early trade on Friday. In the US, the Dow Jones Industrial Index ended 0.70 per cent higher on Thursday.