Nikkei opened almost flat Tuesday morning after a national holiday. At 9:15 a.m., the 225-issue Nikkei Stock Average stood 6.25 points, or 0.07 percent, down from Friday at 9,153.14. The broader Topix index of all First Section issues on the Tokyo Stock Exchange was up 0.71 points, or 0.09 percent, at 757. 59.