With a hardline nationalist challenger enjoying a clear lead from Sunday's first round of presidential elections, Serbia is set for a February 3 run-off increasingly portrayed as a "referendum" on ties with the west.

Tomislav Nikolic, acting leader of the Serb Radical party, won 39.4 per cent of ballots cast, compared with 35.4 per cent for the strongly pro-European Union incumbent Boris Tadic, according to the Centre for Free Elections and Democracy (CESID), a reliable independent monitoring organisation.