TOKYO, Jan 7 (Reuters) - Japan's Nikkei share average fell for a second day on Tuesday after data showing a slowdown in growth in the U.S. services sector made investors wary of taking on risk, while Fast Retailing Co sagged on weaker-than-expected monthly sales. The Nikkei ended down 0.6 percent at 15,814.37 in choppy trade. During the morning session, it briefly flirted with positive territory. Index-heavyweight Fast Retailing shed 1.0 percent and had the biggest downside impact on the Nikkei index, taking about 16 points from the index after its same-store sales at its Uniqlo clothing chain in December disappointed the market. The Topix dropped 0.7 percent to 1,283.25. The JPX-Nikkei Index 400, which started trading on Monday, shed 0.7 percent to 11,590.20. On Monday, data from the Institute for Supply Management showed the pace of growth in the U.S. services sector slowed for a second straight month in December with business activity expanding at a lower rate and new orders contracting.