Khalilzad tweeted that he and Mullah Abdul Ghani Baradar, a cofounder of the militant group, had held a "working lunch" ahead of a fresh round of talks with the Taliban as the US seeks a way out of its longest war, AFP reported.
The arrival in Qatar late Sunday of Baradar, seen as close to Taliban leader Haibatullah Akhundzada, has helped fuel speculation of a breakthrough.
Marathon talks last month saw the two sides walk away with a "draft framework" that included a Taliban vow to prevent Afghanistan from becoming a haven for international terror groups.
There was no accord on a US withdrawal or a cease-fire, however, issues which have derailed attempts at peace talks in the past, while the government in Kabul has voiced increasingly loud fears it was being sidelined from the talks.
"Arrived in #Doha to meet with a more authoritative Taliban delegation. This could be a significant moment. Appreciate #Qatar for hosting & #Pakistan in facilitating travel. Now the work begins in earnest," Khalilzad had tweeted.
He later posted: "Just finished a working lunch with Mullah Beradar and his team. First time we've met. Now moving on to talks."
Baradar was arrested in Pakistan in 2010, but was released in October and named as head of the Taliban's political office in Doha.
He was long considered the number two to Taliban chief Mullah Omar, who died in 2013. Now he is one of several deputies to Akhundzada, along with Omar's son and the head of the Haqqani network, Sirajuddin Haqqani.
Meanwhile the government in Kabul continued to voice concerns Monday over being sidelined in the negotiations.
The Taliban have steadfastly refused to negotiate with the Kabul government, whom they dismiss as "puppets".
"The Taliban are still not ready to talk to Afghan government, but we are ready. We think that Taliban's dishonesty is the only obstacle," said Abdullah Abdullah, the country's de-facto prime minister, in a televised address Monday.
"We are flexible and ready to make a team that is acceptable to all."