Japan's key Nikkei index opened lower Monday, dropping below the 10,000 line for the first time in nearly two months, as the uncertainty about global economy clouded investors' sentiment.

In the first 15 minutes of trading, the benchmark 225-issue Nikkei Stock Average was down 59.69 points, or 0.59 percent, from Friday to 9,997.40, falling below the psychologically important 10, 000 threshold for the first time since Dec. 11, when it logged an intraday low of 9,916.21.

The broader Topix index of all First Section issues on the Tokyo Stock Exchange lost 4.72 points, or 0.53 percent, to 887.06. The Second Section also lost ground.