Antonio Guterres, Portugal's former prime minister who was the UN's refugee chief for 10 years, took the lead in the first straw poll vote on Thursday to pick the next UN secretary-general, diplomats said.
Slovenia's former president Danilo Turk came second in the secret vote by the UN Security Council to choose a successor to Ban Ki-moon.
"Guterres is the man to beat," a Security Council diplomat told AFP. "He has done very well."
The 15 ambassadors including those from the powerful permanent five -- Britain, China, France, Russia and the United States -- each rated the candidates with ballots marked "encourage," "discourage" or "no opinion."
Guterres won the top score of 12 "encourage" votes, while Turk was close behind with 11 encouragements, according to diplomats.
Turk received two "discourage" votes.
Council members are expected to meet again, possibly as early as next week, for a second round of straw polls, with a final nominee expected to emerge by October.
There are currently 12 candidates in the race, six of them women, but diplomats said some were expected to withdraw following the result of the first round.
One candidate received 11 "discourage" votes from the council.
Fluent in several languages, the 67-year-old Guterres impressed UN diplomats when he appeared at the first-ever public hearings for candidates in April.
"We think the results reflect very much the performance in the hearings," said a council diplomat.
Turk, 64, was Slovenia's president from 2007 to 2012 and also served as the country's first UN ambassador in 1992.
After his posting as Ljubljana's envoy, he was appointed UN assistant secretary-general for political affairs, a post he held until 2005 before returning to Slovenia, where he taught law.
Among the other contenders were Argentina's Foreign Minister Susana Malcorra and New Zealand's ex-prime minister Helen Clark.
US Ambassador Samantha Power told reporters ahead of the vote that the United States would back a candidate with "great leadership and management skills."
"This could not be a more important job and it could not be a more important time to choose the best possible leader for this organization on which so much depends and so many depend," Power told reporters ahead of the vote.
Britain's envoy called for a "strong" secretary-general to be elected, while France stressed language skills, vision and decisiveness as important criteria for the job.
Power said the next UN leader will have "so much to do" from confronting terrorism to combating climate change, advancing the UN's development goals and making sure UN peacekeepers "act on behalf of civilians."
Council members are facing calls to pick the first woman secretary-general after eight men in the job, and to give preference to a candidate from eastern Europe, the only region that has yet to be represented in the top post.
"It is high time for a woman," British Ambassador Matthew Rycroft said, but he added that there were "very, very strong men" in the race and that Britain will not use its veto to block a man from winning the post.
For Britain, supporting a candidate from eastern Europe is "the least important criteria here," he said.
French Ambassador Francois Delattre said the best candidate to become the world's number one diplomat must have "leadership, diplomatic skills and multilingualism."
"The next secretary-general will have to have the skills, decisiveness and vision to lead the organization in these testing, and troubled times," added Delattre.
The council will ask the General Assembly to endorse the nominee, who will begin work on January 1.