Olympiakos Piraeus opened up a four-point lead at the top of the Greek Super League on Sunday as rivals Panathinaikos slipped to their first defeat of the season.
Former Liverpool winger Albert Riera struck the only goal of the match for Olympiakos in the second minute of their home game against Aris Salonika.
Panathinaikos suffered an early setback against AEK Athens in the capital's big derby.
Algeria striker Raffik Djebbour made the most of a Josu Sarriegi error in the Panathinaikos defense and fired in a low shot that was too good for Alexandros Tzorvas.
Panathinaikos produced a number of chances, their best falling in the second half to an unmarked Gilberto Silva who blasted over from a couple of metres out.
The 1-0 defeat put Panathinaikos just one point ahead of AEK Athens who appear to have settled under new coach Manolo Jimenez.
Newly promoted Kerkyra are in fourth place after Denis Epstein and Ilias Ioannou scored in a 2-0 win at Panserraikos.