The Nikkei 225 index at the Tokyo Stock Exchange opened down 51.01 points at 8,644.97 after the Dow Jones Industrial Average pulled back from triple-digit advances and closed up 0.65 percent in New York.
Raising the stakes three days before Europe's leaders are to meet on the deepening debt crisis, S&P on Monday warned the 15 eurozone members of possible cuts to their sovereign credit ratings.
The agency said it would complete a review of their ratings "as soon as possible" following the EU summit in Brussels on Thursday and Friday.
"Systemic stresses in the eurozone have risen in recent weeks to the extent that they now put downward pressure on the credit standing of the eurozone as a whole," the ratings agency said in a statement.
Cyprus and Greece, with their ratings already cut to just above or at junk bond level, were not affected by the warning.
The euro fetched $1.3391 and 104.22 yen in early Asian trade, compared with $1.3394 and 104.27 yen in New York late Monday.
The dollar was also almost flat, trading at 77.83 yen.