TOKYO, Dec 11 (Reuters) - Japan's Nikkei share average declined on Wednesday, which traders attributed to a reduction in investor exposure to risky assets as they waited to see if the U.S. Federal Reserve will taper its stimulus soon. The Nikkei ended down 0.6 percent at 15,515.06, extending its declines into a second day. The Topix fell 0.5 percent to 1,250.45. Yahoo Japan Corp outperformed the market, soaring as much as 6.6 percent to a 7 1/2-year high after JPMorgan started its coverage at 'overweigh.' The gains built on a rally over the past two days of more than 10 percent after Goldman Sachs added it to its 'conviction buy' list. Yahoo ended up 4.7 percent and was the fourth most traded stock by turnover. Internet gaming firm Gree Inc surged as much as 20 percent before it ended up 2.3 percent after Goldman Sachs upgraded its rating to 'neutral' from 'sell'. It was the second most traded stock by turnover. On the losing ledger, Mixi Inc nosedived 17 percent, giving up all of its previous session gains, after Goldman Sachs's downgraded its stock to 'sell' from 'neutral'.