The radical leftist SYRIZA party is racing neck-and-neck with the conservative New Democracy party ahead of the election, which could decide if Greece stays in the euro zone and spread turmoil across global financial markets.
SYRIZA leader Alexis Tsipras is threatening to tear up the punishing terms of the 130 billion euro bailout that is keeping Greece from bankruptcy.
"If the radical left wins - which cannot be ruled out - the consequences for the currency union are unforeseeable," Juncker, head of the group of euro zone finance ministers, told Austrian paper Kurier.
"We will have to speak to any government. I can only warn everyone against leaving the currency union. The internal cohesion of the euro zone would be in danger."