ATHENS (Reuters) - Former Greek Prime Minister Alexis Tsipras played down fears on Sunday that a snap election in two weeks would produce a fractured result, saying there were enough undecided voters to produce a clear winner on the day.

Tsipras resigned last month to make way for the election, hoping to secure a new mandate for a tough austerity program he agreed in exchange for an 86 billion euro bailout with the cash-starved country's creditors.

But having started out as the clear frontrunner, his leftist Syriza party's poll lead has collapsed in the past days, making for an unexpectedly close contest against his main rival, the conservative New Democracy party.

The prospect of a fractured result after the Sept. 20 vote has stoked fears of yet more political instability in a country hit by years of instability and recession, and raised the prospect of Greece having to go to the polls again.

"There is 15-20 percent of undecided voters right now. In simple maths, this means that the party that will come first, if it doesn't secure an outright majority at the parliament, will be very close to it," Tsipras said in an interview with Skai television that was broadcast on Sunday.

"So a government will be formed."

Tsipras stormed to power in January promising voters an end to austerity and that he would redefine the relationship Athens has with its international lenders - the European Commission, the International Monetary Fund and the European Central Bank.

But with the economy on the brink of collapse, its banks shut and the chance of a euro exit looming ever closer, Tsipras eventually capitulated to the creditors' demands.

On Sunday, he again defended that decision, saying the country would have been worse off without the bailout, and also ruled out forming a national unity government with New Democracy if the election proved inconclusive.

"There is no room for cooperation in government because the differences in our policy programs are very big and because we know very well that we don't share the same target," he said.

Although he was forced to back down in negotiations with the creditors, Tsipras has at least secured the prospect of some form of relief for the country's ballooning debt, although its largest creditor, Germany, has ruled out any form of "haircut".

Tsipras, who never wears a tie and whose government became known for its informal style, said he would don a tie if he secured debt relief. Asked when this will happen, he said:

"We will have this very important development over the debt by the end of the year. We will have a difficult year but I am optimistic."

Tsipras will make a major campaign speech, which could include new policy announcements, in the northern city of Thessaloniki on Sunday evening.

(Writing by Matthias Williams; Editing by Alison Williams)