Serbian Radical Party leader Nikolic is currently neck-and-neck in the opinion polls with pro-European incumbent Boris Tadic ahead of the February 3 presidential runoff. Nikolic gained a five-point advantage in the first voting round on January 20.
Yelena Guskova, who heads the Center for Studies of the Modern Balkan Crisis at the Institute of Slavonic Studies of the Russian Academy of Sciences, said: "I believe Nikolic has a stronger chance. But it is difficult to predict what will happen, because in any election administrative resources play a major role."
The "Russian factor" will have a significant influence on the result, she said.
Guskova said that Russia will cooperate with whichever candidate wins the election, as relations with Belgrade are of prime importance to Moscow.
"It is important to Russia that the Serbian people make their choice, and we will cooperate with any president," she said.
Nikolic, widely seen as an anti-Western far-right figure, concluded a two-day visit to Moscow on Wednesday. During talks with the speakers of Russia's upper and lower houses of parliament, he said he considered Russia to be Serbia's main ally, and that if elected he would seek to expand economic and political ties with the country.
Tadic visited Moscow last week, and oversaw the signing of deals on the purchase of a controlling stake in Serbia's state-owned oil monopoly Naftna Industrija Srbije by Russia's Gazprom Neft, and on the construction of the Serbian section of the South Stream gas pipeline system.
Russia, Serbia's long-time ally, has been an outspoken supporter of Belgrade's position on the breakaway province of Kosovo. While most Western powers have said they will back a unilateral declaration of independence by Serbia's predominantly Albanian province, Russia insists that Serbia's sovereignty and territorial integrity must be respected. The issue will be on voters' minds as they go to the polls on Sunday, with Kosovo expected to declare independence within weeks.
Both Tadic and Nikolic are strongly opposed to Kosovo's independence, but Nikolic has promised a tougher stance on relations with the European Union if it recognizes the breakaway province's statehood.
Kosovo has been a UN protectorate since the NATO bombing of the former Yugoslavia ended a conflict between Albanian and Serb forces in 1999.
