A London perspective on next week's G20 summit inevitably places Britain, and the Prime Minister Gordon Brown, at the centre. But among those countries briefly orbiting the British sun there are two – the United States and China – that could, almost by themselves, dictate the future of the world. What is more, recent days have supplied ample evidence of the potential for friction between them. Already they seem to be circling each other warily, probing their comparative strength.

The immediate source of discord, and the one mostly closely related to the financial crisis, is China's eager embrace of a proposal for a new global reserve currency. The suggestion was first mooted by Russia in its draft summit proposals, but China's official expressions of support gave it a life, and a plausibility, it would never otherwise have had. The call reflects a widespread view that the status of the dollar as the premier reserve currency gives the United States an advantage that is inappropriate and unjust in the new "multi-polar" world.

It had been thought in some quarters that the euro would in time challenge the pre-eminence of the dollar. And to an extent it already has. It has also shown itself surprisingly robust in the face of the current storms. If this crisis has shown anything, however, it is that the US dollar is still seen as a safe haven, even when the US economy looks as shaky as it does.

China's enthusiasm for a global reserve currency, however, is also part of a continuing quarrel between Beijing and Washington about the role of China in the financial crisis – in building up vast reserves of dollars against cheap exports – and Beijing's resistance to revaluing its own currency. A source of friction during the Bush years, the revaluation question was raised by the Obama administration in its first week.

It is not hard to see how a new global reserve currency might offer another way. So seductive was the idea, indeed, that the US Treasury Secretary, Tim Geithner, seemed initially to entertain the possibility – precipitating a fall in the dollar and a rapid about-turn. The fumbled US response, however, hints at an opening for China to exploit in future.

As it happened, the re-emergence of the old currency dispute in this new form coincided with increased friction in the other main area of potential rivalry: defence. Earlier this week, the Pentagon released a report charting what it saw as China's rapid progress in modernising its military and asking, apprehensively, how it might use its "expanding military power".

China had, the report said, more than doubled defence spending in the past eight years and acquired an armoury of advanced foreign weapons to the point where the military balance was being changed, not just in the Asia-Pacific region, but beyond. The risks were underlined by reports that the Chinese navy had harassed a US surveillance ship in the South China Sea – an incident eerily similar to one early in George Bush's presidency, when a US spy-plane was forced to crash-land in China.

It would be premature to talk of the financial crisis heralding the terminal decline of the United States as a global power and the acceleration of China's rise; still more exaggerated to forecast a new superpower conflict as a consequence. But it is already possible to catch glimpses of the economic and military challenge China will eventually present – unless thrown off course by civil unrest – and the epic shifts that could result. To be forewarned is to be fore-armed.