Panathinaikos suffer shock home defeat
Panathinaikos, who began the evening two points adrift of the leaders, failed once more to sparkle on home soil against a well-organised Volos, who clinched all three points thanks to Vicente Monje's 78th-minute strike.
The Argentine slotted in from close range after 'keeper Alexandros Tzorvas had parried a Javier Umbides shot.
The result left Panathinaikos on 31 points after 15 matches, meaning Olympiakos can move five points clear ahead of the short winter break if they win at bottom-club Larissa on Sunday.
Third-placed PAOK Salonika and fourth-placed AEK Athens, who are both 10 points behind the leaders, also play on Sunday.
Elsewhere on Saturday, Kavala moved into fifth spot on 21 points despite being held 1-1 at home to Atromitos.
Ergotelis continued their recent good form, moving up to seventh place on 20 points courtesy of a 2-0 home win over Panserraikos, the goals coming from Mario Budimir (17) and Tomasz Wisio (85).