TOKYO, Oct 23 - The Tokyo Nikkei average fell more than five percent by mid-afternoon on Thursday -- on track for its biggest percentage drop since the September 11 attacks -- after a steep fall on Wall Street, with high-techs sold by jittery investors ahead of Sony Corp's earnings results. As of 0509 GMT, the Nikkei average <.N225> was down 4.94 percent at 10,352.12, breaking below 10,500 yen for the first time since October 9. It fell as low as 10,311.72 at one point. If the Nikkei closes at its late afternoon levels, it would mark the biggest one-day percentage drop since September 12, 2001. The broad-based TOPIX index <.TOPX> fell 4.86 percent to 1021.56, with losses heavy across a range of sectors.//www.reutes.com