The meeting this weekend in California between US President Barack Obama and his Chinese counterpart, Xi Jinping, marks many ''firsts'' and holds much promise.
It will be the first time the two have met as presidents. (They met last year in Washington when Xi was China's vice-president and it was widely assumed he would soon take the mantle of power.)
It is also rare for a Chinese president to make a trip to the United States this early in his term - less than three months since he was installed as the country's head of state. And one would need to go back to the Nixon-Mao meetings of the early 1970s to recall a time that the leaders of the two countries met in relatively unscripted sessions to discuss the US-China strategic relationship and its future.
No doubt these are positives. It is essential that these two leaders hold open-ended and wide-ranging discussions. It is also critical that they do so this early in Xi's tenure, in a ''no neck-tie'' format and without the many constraints an official state visit would impose.
But at the same time, the unusual and promising nature of the talks in no way lessens the challenges the two men face to put the US-China relationship on a sturdier footing for the future.
Indeed, given the hype and expectation this meeting has generated in the US, in China and elsewhere, the pressures are even greater to show that some significant outcomes or ''deliverables'' emerge from this important meeting.
But for several reasons, we should temper our expectations.
To begin, the most important items for discussion simply do not lend themselves to basic policy adjustments and deliverables. The biggest question to discuss will be how to improve strategic trust between the two countries.
Put more simply, the two presidents will need to convey credible and reassuring messages to one another - both in tone and substance - on how the two countries can co-exist and mutually benefit, especially as their interests increasingly intersect in the Asia-Pacific region.
If that were not difficult enough, the two men have a lengthy catalogue of specific concerns to discuss, conversations which may - or may not - contribute to building strategic trust.
Finding common ground on such issues as improving economic growth both at home and abroad, mitigating climate change, and combating piracy may be the easy part.
More difficult will be joint solutions emerging from what diplomats call ''frank conversations''. President Xi will raise tough questions on American military activities and long-term intentions around China's periphery and whether the US can agree to ''a new model of great power relations'' (read: treat China as an equal).
Conversely, President Obama will want to hear his counterpart explain what China will do with its growing power.
Will China resolve its territorial disputes peacefully? Can it abide a continuing strong American role in the region? When will China rein in its cyberhackers? Other tough talk on both sides will have to resolve differences over North Korea, Syria, Iran, and much more.
But even if the two leaders genuinely seek and unexpectedly find common ground on some of these issues, challenges of a different sort still lie ahead for both of them: translating goodwill to action in the face of significant domestic antagonism. While it seems that Xi has consolidated his power comparatively quickly, he is still new to the job.
Conservative and nationalist elements in the Chinese body politic in particular will be watching to see if their leader stands up to the American President. Xi will need to strike a delicate balance: asserting Chinese interests and concerns and not seeming to be too accommodating, while at the same time not alienating his counterpart with a lack of compromise.
President Obama's task is no easier, but for different reasons. Given his domestic situation - serious economic and fiscal challenges and a well-entrenched opposition - he will be hard-pressed to convince Americans of the need to become a closer partner with the Chinese for long-term gain.
While the ''moderate middle'' of the foreign policy elite would agree with this approach, look instead for critics on the left and right to strongly question the value of the meeting, particularly as there is unlikely to be any immediate and tangible ''win'' coming out of these discussions.
Regardless of these obstacles, this meeting will fundamentally define the Obama-Xi relationship and US-China relations for years to come. Given those stakes, the most important outcome over the weekend will be something all but incalculable: the personal chemistry between the two presidents.
If President Obama, known for his cool demeanour, warms to the new Chinese leader, if Xi Jinping can get beyond well-worn statements of principle and connect with the US President, then there can be some hope for a new beginning for the world's most important strategic relationship.
Professor Bates Gill is chief executive of the US Studies Centre at The University of Sydney.