On February 3, however, voters flowed to the ballot booths in unexpectedly high numbers (67 percent turnout) to give the win to Tadic and his more moderate, pro-European stance.
Final results have yet to be announced. Tadic currently leads with 51 percent of the vote. Nikolic, who received 47 percent, conceded defeat late on February 3 when preliminary results showed him just two percentage points behind.
"Based on results we have counted so far, I believe that Boris Tadic has most likely won this election," Nikolic said. "If this result is confirmed, I am using this opportunity to congratulate him."
Nikolic, an ultranationalist who faced off against Tadic in the 2004 presidential election with similar results, had hoped growing dissatisfaction with Serbia's stagnant economy and the imminent independence of the ethnic-Albanian breakaway province of Kosovo would draw votes away from Tadic and his pro-European stance.
Instead, many view the vote as a referendum on Serbia's EU aspirations -- including Tadic, who praised voters for highlighting the country's "democratic potential."
"I would like to congratulate all citizens of Serbia who, by taking part in this election, showed that Serbia has a great democracy, a European democracy," he said. "In a way, we have shown the European countries, members of the European Union, what kind of democratic potential we have here in Serbia and I am asking for applause for all citizens of our country, for all Serbia."
The result comes as a relief to the European Union, which is eager to keep Serbia in its sphere of influence -- particularly at a time when Russia's influence in the Balkan region is on the rise. Belgrade has until February 7 to sign a so-called "interim agreement" mapping out closer ties with Brussels.
EU foreign-policy chief Javier Solana has praised the results and said he hoped Belgrade would now move "as rapidly as possible" toward European integration.
"I think that the results -- for me at least -- signaled the wish of the majority of the people in Serbia who want to continue the path towards Europe, and I'd like to say Europe is very happy with that," Solana said.
Kosovo Issue Still Looms
Russia, too, is believed to view Tadic as a preferable partner to the unpredictable Nikolic. But it is deeply opposed to Kosovo independence and will do its best to block any attempts by Brussels to soften Belgrade on the issue.
Duma Deputy Konstantin Zatulin said the West and Russia should come together in welcoming Tadic's reelection, but warned that Brussels should "review" its stance on Kosovo before its unilateral declaration of independence, which is considered to be imminent.
Both Tadic and Nikolic oppose independence for Kosovo, but Tadic has vowed to pursue EU membership even if Brussels recognizes Pristina's independence.
Sabine Freizer, director of the Europe program at the International Crisis Group, tells RFE/RL that the weeks and months ahead will see Moscow's continued opposition to Kosovo independence, which it fears will set a precedent for its own separatist conflicts.
"Russia has made it clear that it will continue to support Belgrade, regardless of who is in office, in trying to push back, or trying to avoid, a Kosovo declaration of independence," Freizer says. "Russia is gaining influence -- economic and political -- in Serbia. I don't necessarily expect this to change after the election."
Another thing not likely to change is the divided nature of Serbian politics. Despite Tadic's win, the election's large turnout also meant record-high votes for Nikolic; his party, the Radicals, has the largest representation in parliament.
Nor are ties likely to be easy for the current governing coalition, which groups Tadic's Democrats with the Serbian Democratic Party of Prime Minister Vojislav Kostunica. Kostunica refused to endorse either Tadic or Nikolic ahead of the runoff -- something that may come back to haunt him now that Tadic has secured his reelection.
Martin Sletzinger, director of the East European studies program at the Woodrow Wilson Center in Washington, says Tadic may now find himself in an unexpectedly strong position.
"Even without the endorsement of his coalition partner [Kostunica], Tadic managed to pull this out," Sletzinger says. "To me, that makes him actually the most important politician in the country, because now he has a mandate. People voted for him personally. The prime minister, who tries to play a kingmaker -- he has no personal mandate."
(RFE/RL's South Slavic and Albanian Languages Service contributed to this report.)