Tokyo's key Nikkei index plunged more than two percent on Friday, hit by a surge in yen and sell-offs on Wall Street amid worries over US economy.In its first trading session of 2019, benchmark Nikkei 225 lost 2.26 percent or 452.81 points to close at 19,561.96 as it was catching up with other markets after New Year's break.The broader Topix index lost 1.53 percent or 22.93 points to 1,471.16.