TOKYO, April 12 (Reuters) - Japan's Nikkei average retreated on Friday after recent sharp gains to an almost five-year high, though it still posted a third straight weekly advance, buoyed by optimism that bold central bank policies would revitalise the economy. The Nikkei closed 0.5 percent lower at 13,485.14. It opened up 0.1 percent at 13,568.25, its highest level since July 2008, before moving into negative territory. For the week, the benchmark rose 5.1 percent. The broader Topix index edged up 0.1 percent to 1,148.57 in active trade.