Tokyo stocks opened sharply lower Friday, with the key Nikkei index briefly sliding over 2 percent, tracking overnight losses on Wall Street as investors locked in gains after recent rapid advances. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average fell 837.23 points, or 1.63 percent, from Thursday to 50,444.60. The broader Topix index was down 29.62 points, or 0.88 percent, at 3,352.10.