Tadic has won 25.34 per cent of the vote and Nikolic 24.99 per cent. Ivica Dacic, the candidate of a coalition led by the Serbian Socialist Party, won 14.24 per cent of the vote.
The runoff, most likely between Tadic and Nikolic, will be held on May 20.
As for the parliamentary election, also held on Sunday, the State Election Commission said, having counted more than 97 per cent of the votes, that the coalition led by Nikolic's party won 24.01 per cent or 73 of the 250 seats in parliament, while the coalition led by Tadic's party won 22.07 per cent of the vote or 67 seats.
The coalition led by Dacic's party won 14.54 per cent of the vote (44 seats in parliament), followed by Vojislav Kostunica's Democratic Party of Serbia with seven per cent of the vote (21 seats).
The coalition Preokret, led by Cedomir Jovanovic's Liberal Democratic Party, won 6.53 per cent of the vote (20 seats), while Mladjan Dinkic's United Regions of Serbia won 5.51 per cent of the vote (16 seats).
The election threshold in Serbia is five per cent. Vojislav Seselj's Serbian Radical Party is among those that did not win enough votes.
The final election results are to be released on Thursday at the latest.