At the start of five days of peace talks in the Swiss city of Geneva, Staffan de Mistura was upbeat about the recent ceasefire now in effect for parts of south-western Syria.
The pause in fighting, brokered by the United States and Russia at the G20 summit in Germany, is centred around the contested city of Daraa.
Mr de Mistura says he is hopeful.
"There have been incidents at the very beginning, there have been some sporadic incidents, but we can still say, and we believe, that it has fairly good chances of working out. The devil is in the detail, there are some issues which may be complicating it, but we believe it has a very strong chance of being implemented."
The ceasefire plan represents the first peacemaking effort in the Syrian crisis by President Donald Trump's administration in the United States.
Russia's foreign minister, Sergey Lavrov, says Russia intends to pursue further cooperation with the United States step by step.
His US counterpart, Rex Tillerson, recently even indicated Russia may have the right approach in resolving the Syrian conflict.
"By and large, our objectives are exactly the same. How we get there, we each have a view. And maybe they've got the right approach and we've got the wrong approach."
But that comment has angered veteran US Republican senator John McCain, who has met with White Helmet rescue volunteers working in rebel-controlled areas of Syria.
Asked by a reporter if the Russians have the right approach, Senator McCain said: "You can't make that up. You can't make that up. These are the same people that use precision guided weapons to strike hospitals in Aleppo where sick and wounded people are. But I've met the White Hats (White Helmets). I know what the slaughter has been like. I know that the Russians knew that Bashar al-Assad was going to use chemical weapons. And to say that maybe we've got the wrong approach?"
World powers are taking different sides in the conflict, with Russia and Iran aligned with Syrian president Bashar al-Assad.
Meanwhile, neighbouring Israel says it is closely monitoring the ceasefire across its border with Syria.
The country's intelligence and transport minister, Yisrael Katz, says Israel will not accept the presence of either Iranian or Hezbollah forces from Lebanon in the area.
"We are against a corridor between Tehran and Beirut, okay? And I think that most of the Arab countries in the region and the United States are against this thing. And the other thing, we are against the establishing of Iranian permanent forces, military forces, in Syria."
Aid groups are warning of major challenges ahead for the millions of civilians forced from their homes in Syria.
And the World Bank says cumulative losses to Syria's gross domestic product since the conflict began are estimated to be nearly $300 billion.
But Staffan de Mistura, with the United Nations, suggests recent developments may help turn the corner towards a solution.
"I think perhaps we are witnessing a phase of simplifying the most complex conflict of our time. It has become, as you know, the most complex conflict because, over the years, several layers have been actually placed, or been placing themselves, over what initiated in a relatively simple but very tense way and then became one layer after the other."
