Vucic's Serbian Progressive Party was on track to take 52.6 percent of votes, with the Socialists -- who have been in the ruling coalition since 2012 -- in second place with 12.9 percent, according to independent observing group CESID.
The far-right Radicals were also set to return to parliament after four years without seats, led by ultra-nationalist Vojislav Seselj who was recently acquitted of war crimes charges arising from the 1990s Balkan conflicts.
The Radicals were expected to win 7.4 percent, CESID said, clearing the five percent threshold and giving them a platform in parliament for their strongly anti-European and pro-Russian views.
But critics saw the vote as an attempt to consolidate power, expressing concerns about Vucic's authoritarian tendencies including curbs on media freedom.
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A former Seselj ally turned pro-European centrist, Vucic said as he voted in Belgrade that he was "almost certain that we'll carry on our EU integration process," expressing hope that voters would choose a "European path".
He was due to speak to make a speech later yesterday night.
Serbia, home to seven million people, opened the first stages in EU membership negotiations in December, although Brussels has said there will be no further enlargement of the bloc until 2020.
"We have elections too often," said retired Jelica Nikolic, 68, in Belgrade, saying she and her husband Radomir were voting more out of duty than conviction.
In the southwestern city of Novi Pazar, Edib Mahmutovic, 40, hoped the victors would "create new jobs that enable us to stay here and not have to look for a better life elsewhere in Europe".