At first glance, a loyal supporter of Iran's Islamic revolution and a Democratic ex-senator have little in common. Yet against the odds, John Kerry and Mohammad Javad Zarif have blazed their way into history.
The tall, lanky American secretary of state, in his expensive tailored suits towers over his shorter, stouter Iranian counterpart, Zarif, in his traditional collarless shirts when they gather for choreographed pictures.
But over months of risky, roller-coaster negotiations to strike an unprecedented nuclear non-proliferation agreement, they have both proved to be steely and at times wily rivals. Even Kerry's broken leg has failed to slow him down.
They are on first name terms, calling each other John and Javad.
And while occasionally they share a joke, the relationship remains business-like, though tinged with obvious respect.
That's perhaps no surprise, given their countries have not had diplomatic ties for more than three decades, and remain at odds over a slew of weighty issues, including Tehran's alleged support for Middle East terror groups.
But it seems John and Javad were the right men for the right season, brought together as the world sought to end rising concerns about Iran's nuclear ambitions.
Already there has been speculation of a possible Nobel peace prize nomination. The deal sealed on Tuesday to curb Iran's nuclear programme will only increase such speculation.
Zarif, 55, was appointed foreign minister by President Hassan Rouhani in September 2013, and was quickly tasked with resuming the nuclear talks with a clear mandate to end the crippling sanctions against his country.
A fluent English speaker with a PhD in international law from the University of Denver, he is a veteran loyalist of the Islamic revolution that toppled the shah in 1979.
At the start of the talks it was Zarif who had the distinctive advantage, having already spent 20 years as a diplomat at the United Nations, where he was also Iran's ambassador from 2002-2007.
American officials on the other hand had had little contact with Iranian counterparts since ties were snapped, although as a senator Kerry, 71, was part of secret US talks in Oman in 2012 to explore the possibility of reopening talks.
Describing Zarif as "brilliant", Iran expert Suzanne Maloney said "he has the ability to sell policies that are fundamentally problematic from the American point of view, in a way that comes off as completely persuasive and appealing."
"It's a misunderstanding to believe that he is somehow more American than he is Iranian," the Brookings Institution expert cautioned.
"He's very much a creature of the Islamic republic, and it's not accidental that he's managed to gain a very high-level position at a crucial time."
But Zarif's long stint in the US earned him the hostility of the ultraconservative camps and he was sacked by incoming hardline president Mahmoud Ahmadinejad.
After six years in academia, Rouhani brought him back to the political fore, offering the promise of rehabilitation.
Deeply religious, he was known to interrupt the negotiations to complete his daily prayers. After which he returned, saying "I fear only one true power."
Despite his faith, Zarif has come under fire from hardliners who suspect him of making too many concessions to the West.
Yet, after marathon all-night sessions in Lausanne in April reached a framework accord, Zarif received a hero's welcome in Tehran.
For Kerry, a practising Catholic, sealing the deal to curb Iran's nuclear programme after almost two years of talks is also a legacy-making victory, halfway through his tenure as America's top diplomat.
It has even greater resonance for the former Massachusetts senator and failed 2004 US presidential candidate after his quixotic bid to strike a long-elusive Middle East peace deal spectacularly collapsed last year.
A breakthrough agreement "between two old adversaries, has got to be a triumph, both personally and professionally," Maloney said.
And while it might not boost Kerry's weight on other thorny dossiers such as Ukraine or the Middle East, "it's going to demonstrate the effectiveness of someone who's simply willing to continue to go the mat on a difficult issue".
Zarif and Kerry first met at the start of the talks at the United Nations in September 2013, when the Iranian diplomat surprised everyone with the huge smile which never seemed to leave his face.
In the months since, he has gained a reputation for being charming and articulate.
Yet, officials say in the negotiating room he has at times grown emotional and even angry, when pushed too hard on something he felt he could not deliver.
Zarif is also one of only a few Iranian officials to have a Twitter account, which remains banned in Iran. He has used that and YouTube effectively to push his message to the US.
His profile includes a compelling quote from the 13th century Persian poet, Saadi, which reads in part: "All human beings are members of one frame; Since all, at first, from the same essence came."