South Africa's minister of tourism Marthinus van Schalkwyk is front-runner to replace Yvo de Boer as UN climate chief, according to sources.
The run-off is between developing country candidates, reflecting their rising status in stalled UN climate talks to agree a successor to the existing Kyoto Protocol, reports Reuters.
De Boer steps down on 1 July after almost four years.
An interview panel has selected a final shortlist of van Schalkwyk and Costa Rica's Christiana Figueres, one source said, adding van Schalkwyk had the support of key countries, reports Reuters.
UN Secretary-General Ban Ki-moon is expected to make the final decision.