Analysts predict investors faced with a US default would try to sell dollars for yuan, forcing China's central bank to either buy up dollars at a time when the government issuing them isn't honoring its obligations or allow a rapid increase in the yuan's value that would hurt exports and worsen the country's credit bubble.

"If China allows the yuan to rise sharply, it could be very risky given the possible asset bubbles in the country," said He Yifeng, an economist at Hongyuan Securities in Beijing.

Japan, meanwhile, is trying to end 20 years of deflation and anaemic growth with a blend of policies named for their chief proponent, Prime Minister Shinzo Abe. "Abenomics" relies on reviving domestic consumption and investment in part by weakening the yen, boosting the earnings and stock prices of giant exporters like Toyota Motors Corp and Hitachi Ltd .

A US default would likely prompt investors to buy yen.

"That would undoubtedly pose a headwind against Abenomics, which has much depended on a weak yen and higher share prices buoyed by the feel-good mood it has generated," said Masamichi Adachi, senior economist at JPMorgan Securities in Tokyo.

Quid pro quo

A default also stands to hamper US recovery and with it a nascent rebound in global exports. A default in the first quarter of 2014 would hit just as Japan's economy girds for an April sales tax increase and as China's economy loses the effects of accelerated public works spending and re-stocking of inventories.

There are no bond markets large enough to give China and Japan an alternative to US Treasuries for the dollars they accumulate selling exports. So the prospect of another US default drama next year is likely to lend new urgency to China's preferred solution: conducting less trade in dollars and more in renminbi.

About 18 percent of China's total trade is settled in yuan and it has registered a sharp increase this year after stagnating for most of last year.

Much of the increase has come in China's trade with economies outside the United States or the European Union, its biggest demand centers, although it accelerated plans to internationalize the currency with agreements this month with Britain and the European Central Bank.

Achieving more trade in yuan, however, means giving China's trading partners a place to invest their renminbi as easily as they invest their dollars in U. Treasuries now.

That means opening China further to foreign investment, a realization that could strengthen the hand of officials advocating faster reform.

"China can only be strong when its currency is a real alternative," said Chan at Credit Suisse. "But to be an alternative you have to have an open market."